gem identification

Magnification

6

Gem Identification

Table of Contents

Subject Page

Loupes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Loupe Lighting Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Gemological Microscopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Focusing the Microscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Microscope Lighting Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Darkfield Illumination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Brightfield Illumination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Reflected Lighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Diffused Lighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Polarized Lighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Examining a Stone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Identifying Clarity Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Surface Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29Inclusions in Natural Gems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32Inclusions in Synthetics and Imitations . . . . . . . . . . . . . . . . . . . . . . 35

Identifying Assembled Stones . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Key Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Key Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

©The Gemological Institute of AmericaCarlsbad, CA 92008

©2003 The Gemological Institute of AmericaAll rights reserved: Protected under the Berne Convention.No part of this work may be copied, reproduced, transferred, ortransmitted in any form or by any means whatsoever without theexpress written permission of GIA.

Printed in the United States.

Cover photos: All by Terri Weimer/GIA

Facing page: Magnification makes this included crystal visible. Its presence proves that the host sapphire is a natural gem.

MAGNIFICATION“I know it’s a ruby—I get the right RI readings, and the dichroscopeconfirms it’s doubly refractive,” said Mike. “But when I look at it underthe microscope, I can’t tell whether it’s natural or synthetic.”

“I know. It’s getting tougher to separate treated natural rubies from syn-thetic ones. But let me take a look,” said Joe. “There’s usually something—some mineral crystals, clouds, or needles—that proves it’s a natural gem.”

Joe examined the ruby carefully under the microscope. “I can see acouple of melted mineral crystals close to the girdle, and that whitishcloud is probably the remains of some growth zoning.”

“But what about that area under the table—doesn’t that look like fluxto you?” asked Mike.

“It looks more like the remnants of heat treatment,” replied Joe.“When you’ve seen as many treated natural rubies as I have, you’ll rec-ognize them quickly. Looking at a lot of them is the best way to becomefamiliar with their characteristics.”

1©2003 GIA. All rights reserved.

Mike Havstad/GIA

Alan Jobbins

A whitish, hexagonal area is a featurethat identifies some heat-treated, naturalMong Hsu rubies.

Like Mike’s ruby, many gems carry signs of their nature in the form ofcharacteristics called inclusions, which are enclosed within a gem or reachits surface from the interior. Some types of inclusions—like mineralcrystals—are remnants of the rocks that natural gems grow in. Others—like curved color banding and platinum platelets—are found in syntheticgems and hint at the processes used by manufacturers to grow them.

Blemishes can help with identification, too. Blemishes are characteris-tics like scratches and abrasions on a polished gem’s surface. And theycan often indicate a gem’s hardness. For example, a fairly soft gem like

6G E M I D E N T I F I C A T I O N

2

John Koivula/GIA

These inclusions look similar, but one is a flux inclusion in a synthetic ruby (above)and the other is a borax inclusion in the partially healed fracture of a heat-treatednatural ruby (left).

Duncan Pay

Dietmar Schwarz

As gem crystals grow, they often trap mineral inclusions from their environment. Theincluded minerals help identify a gem as natural. Magnification at 50X reveals stalk-like green amphibole crystals in this natural emerald.

Inclusion—A characteristicenclosed within a gemstone or reaching its surface from the interior.

Blemish—Characteristic or irregu-larity confined to the surface of apolished gemstone.

demantoid garnet (Mohs 6.5) often has abraded facet edges, while ahard gem like corundum (Mohs 9) usually doesn’t.

Magnification can help you determine if a gem is treated or if it containsinternal fractures, vulnerable cleavages, or other structural defects. It’s alsoan important tool for separating natural gems from synthetics. This is avital separation because there’s such a large value difference between manysynthetic gems and their natural counterparts of equivalent quality.

For example, it’s easy to separate emerald from other green gems likechrome tourmaline, chrome diopside, green sapphire, and peridot using

MAGNIFICATION

3

Alan Jobbins

You’ll usually see abraded facet junctions on gems that don’t rate very high on theMohs hardness scale. This demantoid garnet shows abrasions on its crown andpavilion facet junctions, along with a prominent horsetail inclusion.

Both by Nicholas DelRe/GIA

This pendant (right) contains diamonds, natural rubies, and synthetic rubies.Magnification reveals gas bubbles in the stone at bottom right (above), indicatingit’s a synthetic ruby.

Magnification is a valuable tool for detecting treatments and for separating natural gems from their synthetic counterparts.

the refractometer because each gem has a very different refractive index(RI). It’s much more challenging to tell if an emerald is natural or syn-thetic. That’s because the physical and optical properties of many naturaland synthetic stones—including emerald—overlap.

Magnification can be a very powerful tool, and the more you practiceusing it, the more skilled you’ll become at recognizing the features thathelp you make a final determination. But it’s also important to keep upwith the latest industry information by reading gemological business andscientific journals.

Gemologists use two types of magnifiers: loupes and microscopes.Loupes are small, easy-to-carry magnifiers that come in a variety offorms. Microscopes are much more sophisticated and capable of fargreater magnification, but they’re much less portable.


4

Terri Weimer/GIA

A gemological microscope is more versatile and provides greater magnification thana loupe, but a loupe is much more portable. GIA Gem Instruments carries a varietyof loupes and microscopes.

Practical experience and up-to-dateknowledge are the keys to using magnifi-cation successfully in gem identification.

You can keep up to date with the constantly changing gem world by reading trade publications like Gems & Gemology.

LOUPESn What’s the most popular loupe for gem identification?

n How do you light a gem to examine its surface?

n How do you light a gem to examine its interior?

A loupe’s portability and affordability make it a versatile tool for gemidentification. You learned how to use a loupe in Assignment 2. Bypracticing the technique outlined there, you can master the loupe andmaximize its effectiveness in the gem identification process.

Some jewelers use an eye loupe that attaches to eyeglasses or fits inan eye socket like a monocle, leaving both hands free to examine astone or to work on a jewelry piece. But today, most gem professionalsuse hand loupes. A hand loupe has a cover that doubles as a handle.

MAGNIFICATION

5

Reporters Press Agency/eStock Photo

The hand loupe’s lens cover serves as a handle when you examine a stone. Thecover protects the lens when the loupe isn’t in use.

Loupes come in powers from 2-power (2X) to 30-power (30X).Under 2X magnification, the diameter of the image is 2 times greaterthan the diameter of the object you’re magnifying; under 30X, theimage’s diameter is 30 times the object’s diameter. The most widelyused loupe in the jewelry industry is the 10X loupe. But not just any10X loupe will do. It must be a good-quality instrument to be useful forgrading and testing gems.

If you look at a gem through a low-quality loupe, you’ll notice thatthe facet edges are in focus at the center of the lens, but appear blurredaround the edges. This is called spherical aberration, and it occursbecause the lens can’t keep the entire image in focus at the same time.

Another form of distortion occurs when a lens can’t focus all thecolored wavelengths of white light at the same point. This effect is calledchromatic aberration, and it causes fringes of color around lines such asfacet edges. If you look at a diamond through such a lens, chromaticaberration might mislead you about its color.

Good-quality loupes cure these distracting optical effects by using threelenses joined together into one unit. One lens acts as a magnifier, anothercorrects for spherical aberration, and the third corrects for chromaticaberration. This kind of a loupe is called a fully corrected triplet loupe.You’ll need a 10X triplet loupe to examine, identify, and grade gems.

Because they’re convenient, portable, and inexpensive, loupes areperfect for buying trips. But their relatively low magnifying power canalso be a challenge. Standard 10X magnification is fine for most gradingtasks, when you have to judge the effects of inclusions on appearance.But at that magnification level, it’s often difficult to identify the inclu-sions that distinguish natural gems from synthetics.


6

Spherical aberration—Blurringaround the edges that occurswhen a lens can’t get an entireimage in focus at the same time.

Chromatic aberration—Color dis-tortion caused by the inability of alens to bring the various coloredwavelengths of light into focus atthe same distance.

Fully corrected triplet loupe—Aloupe that contains a three-partlens that magnifies and correctsfor spherical and chromatic aber-ration.

Terri Weimer/GIA

These are three of the 10X loupes available from GIA Gem Instruments. Each onehas its advantages. The smaller hand loupe is compact, while the larger one gives agreater field of view. The darkfield loupe at the top provides darkfield illumination,which makes it easier to identify many inclusions.

A fully corrected 10X triplet loupe is anessential gemological tool.

Working distance—the distance from the lens to the surface of theobject—also affects a loupe’s usefulness. It’s determined by the loupe’sfocal distance, which is the distance from the surface of the lens to apoint that’s in sharp focus.

The higher the magnification, the shorter the focal distance andworking distance. A 10X loupe focuses when an object is one inch away.A 20X loupe doubles the magnification, but cuts the focal distance inhalf, which means it focuses when the object is half an inch away. Thisalso cuts the working distance in half, leaving less of a margin beforethe stone or its characteristics are out of focus. At 30X, the workingdistance is even smaller.

MAGNIFICATION

7

Terri Weimer/GIA

An uncorrected loupe shows both spherical and chromatic aberration.

Focal distance—The distancefrom the surface of a lens to apoint that’s in sharp focus.

Peter Johnston/GIA

A fully corrected triplet loupe uses a three-part lens to correct both spherical andchromatic aberration.

This makes loupes with magnifications higher than 10X more difficultto use effectively. The higher the loupe’s magnification, the closer youhave to get to the gem, and the harder it is to focus on an individualfeature within the stone.

The shortened working distance at higher powers also leaves lessroom for lighting and makes it more difficult to light a stone effec-tively. Another disadvantage is that it creates a shallower depth offield. Depth of field is the distance that’s sharp and clear in front ofand behind the point—such as a small inclusion—that you’re focusingon. With higher-power loupes, the depth of field is very small. To keepan object in focus, you need to keep both the loupe and the stone asstill as possible.

Another consequence of higher magnification is that the area of thegem that you can examine—the field of view—becomes smaller.Despite these limitations, the loupe can be an amazingly revealinginstrument.


8

Peter Johnston/GIA

When you use magnification to examine a gem, you have to consider the workingdistance from the lens to the gem, the focal distance from the lens to the character-istic you’re examining, and depth of field, which is the area in front of and behindthe object you’re examining. When you switch to higher magnification, you shortenthe working distance, focal distance, and depth of field.

Depth of field—The distancethat’s clear and sharp in front of and behind the point you focus on.

depth of field

focal distance

working distance

VIEWER

DEPTH OF FIELD

LOW MAGNIFICATION HIGH MAGNIFICATION

LOUPE LIGHTING TECHNIQUES

There are two basic types of lighting you can use to examine transparentgem materials with a loupe. One lets you examine the gem’s surfacewhile the other lets you see internal inclusions more clearly.

The first is reflected lighting—strong, direct lighting reflected off agem’s surface. Under reflected light, features like surface-reachingfractures, abrasions, and cavities stand out strongly against the gem’spolished surface.

You can use almost any strong light source, such as a desk lamp, ahigh-intensity lamp, or a fiber-optic light source like a FiberLite. Holdthe stone face-up with tweezers and position the light source and thestone so the light reflects off the stone’s surface. Tilt the gem until eachfacet in turn shows a bright, shiny, reflective surface. Examine the stoneface-up first and then keep turning the stone until you’ve examined itfrom every side.

The second technique—darkfield illumination—lets you see into agem’s interior. With this form of illumination, inclusions within the gemstand out strongly against a dark background.

MAGNIFICATION

9

Eric Welch/GIA

Darkfield lighting lets you examine the interior of a transparent stone. You can createit by taping a sheet of black paper to the back edge of a lampshade.

Reflected lighting—Illumination ofa gem’s surface by reflecting lightfrom it.

Darkfield illumination—Lighting ofa gemstone from the side againsta black, non-reflective backround.

Reflected light is best for examining astone’s surface.

Darkfield illumination works best forexamining the interiors of transparentstones.

You can use a desktop lamp and black paper to create darkfield illu-mination for a standard 10X loupe. Follow these steps:

1. Use tape strips to hang a sheet of black, non-reflective paper from theback edge of the lampshade.

2. Turn the room lights off. Turn the lamp on, and direct the lightstraight down.

3. Hold the stone face-up with tweezers. Position the stone so the girdleplane lines up approximately with the front edge of the lampshade.

4. Examine the stone’s interior against the black background, adjustingthe stone’s position to find the best view. Keep the stone in the lightand the loupe out of the light. This directs light in from the sides ofthe stone without creating too many distracting reflections off thecrown.

If you’re trying to distinguish diffusion treatment, a third type oflighting—diffused lighting—can be helpful. You can create diffusedlighting by placing a sheet of translucent white material, such as a facialtissue, between the light source and the stone.

Using a loupe, you can perform almost any basic magnification test.But for more powerful magnification needs, the gemological microscopeis the instrument of choice.

GEMOLOGICAL MICROSCOPESn Why is a binocular microscope the best choice for gem

identification?

n What’s the best way to focus a gem microscope?

n What are the various types of lighting needed for examining gems?

With its sophisticated optical system, sturdy construction, integratedlighting, and greater working distance, the gemological microscope canalmost always help you identify treatments and make the vital separationbetween natural and synthetic gemstones.

Most microscopes designed for grading or identifying gems arebinocular, which means they have two sets of lenses. The binocularoptical system has a great advantage over the monocular system, whichhas only one lens set. The binocular system produces a three-dimensionalimage with normal orientation. This makes manipulating the gem mucheasier. Most monocular systems produce images that are flat, upsidedown, and reversed.

There are four basic parts to a typical binocular gemological micro-scope. The housing at the top of the microscope that contains the opticalcomponents is called the pod. You can move the pod up and down tochange the instrument’s focus. You do this by turning the focus-controlknob on the instrument’s arm.


10

Eric Welch/GIA

You can create diffused lighting by taping a translucent white sheet to thefront of a lampshade. It can help youdetect the characteristic color zoning indiffusion-treated corundum and curvedcolor banding in flame-fusion syntheticcorundum.

Pod—The housing for a micro-scope’s optical system, also calledthe head.

Directly below the pod, there’s a working platform called the stage,where you place the gem for examination. Beneath that, there’s housingfor a light source called the light well. Finally, there’s the base, whichsupports the microscope and contains the electrical controls.

The pod contains a complex system of lenses and prisms. The eye-piece lenses are called the oculars, and the lenses nearest the stone arethe objectives. Most microscopes have eyepieces that you can adjust forindividual comfort. Some are equipped with plastic or rubber eyecupsthat help eliminate extraneous light, keep your eyes at the correct distancefrom the oculars, and make the microscope more comfortable to use.The eyecups are removable, and some gemologists, especially thosewho wear glasses, prefer to work without them.

MAGNIFICATION

11

A binocular microscope—like this one from GIA Gem Instruments—is a superior toolfor any gemological task that requires magnification.

Stage—A microscope’s workingplatform.

Light well—Housing for a micro-scope’s light source, locatedbelow the stage.

Base—The support platform thatcontains a microscope’s electricalcontrols.

Oculars—The eyepiece lenses ona gemological microscope.

Objectives—The lenses nearestthe stone on a gemological micro-scope.

pod

stage

light well

base

oculars

objectives

GEMOLOGICAL MICROSCOPE

zoom adjustment

focus adjustment overhead light

rheostat forlight well

overhead light switch

You can determine the power of a microscope’s magnification bymultiplying the power of the oculars by the power of the objectives.For example, 10X oculars and 2X objectives give 20X magnification.The typical range is 10X to 30X or 45X, but some models go up to70X or more.

With some gem microscopes, you change magnification powers byswitching oculars. With others, you turn the objectives to one of severalsettings (1X, 2X, or 3X, for example). But the most versatile models letyou zoom smoothly from one power to another. The zoom adjustment isusually on the side or top of the pod. It can be a single knob or a pair ofknobs, with one on either side of the pod. A calibrated dial displays themagnification of the objectives.

Many models let you attach a doubler, which is a 2X lens that screwson under the objectives, doubling the power. You can accomplish mostgem identification with 10X to 45X magnification. Very few identifica-tions require more than 90X.

While higher magnification decreases the depth and width of field ina microscope, its depth and width of field are much larger than a loupe’sto begin with, so there are fewer practical problems. As with a loupe,higher magnification makes it more difficult to light the stone properly,but the lighting systems built into many gemological microscopes provideeffective illumination at higher powers.

The microscope’s stage has an opening that allows light to passthrough from the light well below. Most gem microscopes have an irisdiaphragm at the top of the stage that you can open or close to controlthe amount of light that comes up from the light well.

The stage might also have a number of sockets where you can attacha stoneholder, which has spring-loaded jaws designed to hold a gem.Because it attaches to the stage, it holds the gem firmly in place and leavesboth hands free to operate the microscope or to record what you see. Ifyou use tweezers rather than a stoneholder, rest them against the edge ofthe light well on the microscope’s stage to hold your gemstone steady.

There’s often an overhead fluorescent light source—a removablesource of daylight-equivalent light—mounted at the front of the stage.

The light well consists of a frosted glass or plastic cylinder inside areflective metal bowl. The microscope’s light source is positioned at thebase of the bowl, immediately below the cylinder. Above that, at the baseof the cylinder, there’s a baffle—a small metal flap that can be openedor closed. When it’s open, light comes directly through the opening tolight the stone from below. When it’s closed, the light is forced to comeup from the sides of the light well rather than through the opening. Asyou’ll see, this is essential for darkfield illumination.

The microscope’s base contains most of its electronics. On the backof the base is a small knob called a rheostat that turns the internal lightbulb on or off and also controls the light’s intensity.


12

Iris diaphragm—A device in themicroscope’s stage that you canopen or close to control theamount of light coming from thelight well.

Stoneholder—A device thatattaches to a microscope’s stageto hold a gem steady.

Baffle—A small metal plate thatyou can close to prevent themicroscope’s light from shiningdirectly through the stone frombelow.

Terri Weimer/GIA

The stoneholder attaches to the side ofthe microscope’s stage and holds thestone in place. The iris diaphragm at thetop of the light well consists of a seriesof metal plates that slide together orapart to control the amount of light com-ing up through the light well. You oper-ate it by moving the lever on the left. Atthe bottom of the light well, there’s ametal flap called the baffle. When it’sclosed, it prevents light from shiningdirectly through the stone. You operateit by turning the knob on the right.

FOCUSING THE MICROSCOPE

It’s important to focus your microscope before you begin each worksession. You should also refocus it periodically during long work sessions.Follow these steps:

1. Turn the well light on. Make sure the baffle is closed and the irisdiaphragm is shut down to a small opening. At 10X magnification andusing both eyes, focus on the edges of the iris diaphragm’s opening,which is near the center of the field of view. Most gem microscopeshave one ocular (usually the left) that you can focus without thefocus-control knob. Remove that ocular and look through the rightocular, keeping both eyes open.

MAGNIFICATION

13

All by Eric Welch/GIA

To set up a microscope’s focus, begin by turning the well light on (top left). Next, make sure the baffle is closed and the irisdiaphragm has only a small opening (bottom left). Then, using 10X magnification and both eyes, turn the focus-control knob tofocus on the edges of the small opening in the iris diaphragm (right).

2. Turn the focus-control knob until the iris diaphragm’s opening is infocus in your right eye. Release the focus-control knob. Don’t touch itagain until after you focus the left ocular.

3. Replace the left ocular. Remove the right ocular and look through theleft ocular, keeping both eyes open. Bring the iris diaphragm’s openinginto focus by turning the ring at the bottom of the left ocular.


14


Continue by removing the left ocular and looking through theright ocular. Keep both eyes open. Turn the focus-controlknob until the edges of the opening in the iris diaphragm arein sharp focus.

Next, replace the left ocular and remove the right ocular. Withboth eyes open, look through the left ocular. Focus on theedges of the iris diaphragm opening by turning the ring at thebottom of the left ocular.

4. Replace the right ocular. Adjust the distance between the oculars foryour comfort. Confirm the focus by looking at the opening in the irisdiaphragm through both oculars at the same time. You should see asingle image, and it should look sharp and three dimensional. If itdoesn’t, repeat the focusing process.

MAGNIFICATION

15

Finally, look through both oculars at the same time to confirmsharp focus on the edges of the iris diaphragm opening.

Continue by replacing the right ocular and adjusting the dis-tance between the oculars to suit your eyes.

MICROSCOPE LIGHTING TECHNIQUES

A microscope offers many more lighting options than a loupe, and dif-ferent types of lighting work best for seeing different gem features. Themost useful techniques are darkfield, brightfield, diffused, reflected, andpolarized lighting.

An overhead light source makes it easier to see surface characteristics.Some microscopes come with a small fluorescent light or let you attachone to the stage for this purpose. You can also use a desk lamp as youwould with a loupe. A fiber-optic system like the FiberLite is a versatilesupplementary light source that’s vital for some separation processes.

Varying the lighting can have a dramatic effect on the visibility andappearance of characteristics, and what you can determine by examiningthem. Surface characteristics visible in reflected light are invisible indarkfield light. And internal characteristics visible in darkfield light areinvisible in reflected light.


16

Eric Welch/GIA

A GIA Gem Instruments FiberLite, like other fiber-optic sources of condensed light,is especially good for horizontal, oblique, and pinpoint lighting. Fiber-optic light isvaluable for identifying treatments and for separating synthetic from natural gems.

DARKFIELD ILLUMINATION

Most gem microscopes have the built-in ability to provide darkfieldillumination for examining inclusions. You just have to turn on themicroscope’s internal light source and close the baffle in the light wellso no light can enter the stone from directly below. Light enters thestone from the sides and a little behind, making inclusions stand outbrightly against a dark background.

The degree to which a characteristic stands out against the surroundinggemstone is called its relief. For example, included crystals are mineralstrapped within a gem as it grows. The brassy, metallic surfaces of pyriteincluded crystals stand out readily in pale emerald, so they’re describedas having high relief.

An included crystal’s relief depends on its RI and often its color,especially compared to the color of the host gem. A cluster of moderatelysized, colorless calcite inclusions in a blue sapphire might be much harder

MAGNIFICATION

17

Peter Johnston/GIA

Most gemological microscopes are designed with a baffle to provide the option ofdarkfield lighting.

With the baffle closed, no light enters the stone from below. Light fromthe sides makes inclusions stand outdramatically against a dark background.

Relief—Contrast between aninclusion and its host gem.

Included crystal—A mineral crystaltrapped within a gem as it grows.

DARKFIELD ILLUMINATION

microscope stage

baffle closedto create darkbackgroundfor stone

light source

stoneholder

to see than a few black chromite crystals scattered around the interior ofa pale green peridot.

Most included crystals are relatively easy to see under darkfieldillumination. Other characteristics, like liquid inclusions—pockets ingems filled with fluids and sometimes other materials—might requiredifferent lighting techniques because they tend to blend into the host


18

Karl Schmetzer

Low-relief inclusions like this spinel crystal in ruby (above) are not as easy to see ashigh-relief inclusions like the black chromite crystal in peridot (left).

Alan Jobbins

Liquid inclusion—Pocket in a gemthat’s filled with fluids and, some-times, gas bubbles and crystals.

John Koivula/GIA

Low-relief inclusions like this one, which contains a liquid, a gas, and a tiny crystal,are common in some emeralds from Colombia.

gem’s background if you use darkfield.

Horizontal lighting is a pinpoint darkfield technique, where you directa narrow beam of light toward the side of the stone. A fiber-optic lightworks best for this type of lighting. You can aim the light straight at thestone or from an oblique angle. When you look at the stone from above,pinpoint crystals and gas bubbles stand out as bright objects.

MAGNIFICATION

19

Both by Eric Welch/GIA

By using a fiber-optic light source to illuminate a stone horizontally and examiningthe gem with the well light both on (left) and off (right), you can see inclusions thatmight otherwise go undetected.

Eric Welch/GIA

Oblique lighting places the illumination from a fiber-optic light at an angle betweenhorizontal and overhead.

John Koivula/GIA

Horizontal lighting reveals minute fluxparticles in a Kashan synthetic ruby.

BRIGHTFIELD ILLUMINATION

Brightfield illumination—sometimes called transmitted light—resultswhen you open the light well’s baffle so the light is transmitted directlythrough the stone to your eye. To keep from being dazzled by the brightlight, close the iris diaphragm so the opening is smaller than the stone.This will create focused, pinpoint illumination. If necessary, adjust theintensity of the light source with the rheostat.

Brightfield illumination makes inclusions look dark and featurelessagainst a bright background, so it works well for seeing low-relieffeatures like curved striae in flame-fusion synthetics. Curved striaeare structures that represent the layers of crystal growth around the


20


With the light well’s baffle open, brightfield illumination transmits light up through a transparent stone to your eye.

You create brightfield pinpoint illumination by keeping the baffle open and closing the iris diaphragm until its opening is smaller than the stone.

John Koivula/GIA

Brightfield pinpoint illumination reveals gas bubbles and curved striae in a flame-fusion synthetic ruby.

Curved striae—Curved growthpatterns seen in flame-fusion synthetics.

cylindrical or rod-shaped boule, which is a typical product of the flame-fusion process.

The brightfield technique works best if you close the iris diaphragmand restrict the light source to a small opening directly under the stone.This lets you see structures like curved striae more clearly.

You can actually create an effect similar to brightfield by rocking andtilting the gem under darkfield lighting to create alternating dark andbright backgrounds. This can be helpful for detecting flash-effect colorsin fillers—seen mostly in fracture-filled diamond and emerald—ordetermining if an inclusion is liquid or solid, transparent or opaque.

MAGNIFICATION

21

Both by Terri Weimer/GIA

By rocking and tilting a stone in darkfield lighting, you can create a brightfield effect.Alternating dark to light can help you detect flash-effect colors in fillers or see if aninclusion is liquid or solid, transparent or opaque.

Both by John Koivula/GIA

Rocking and tilting this emerald to alternate dark (above) and bright (right) back-grounds revealed an orangy yellow to blue flash effect in the filler.

Use brightfield illumination to detect low-relief features like curved striae.

REFLECTED LIGHTING

Darkfield, brightfield, and diffused lighting make many inclusions easierto see, but reflected light works better for surface characteristics andsome types of inclusions. To examine a gem’s surface characteristics,you need to position the light source, which is usually the microscope’soverhead light unit, so the light reflects off the gem. The light shouldstrike the gem’s surface at close to a 90° angle—this is called verticaloverhead illumination.

Thin, flat inclusions—like the thin films seen in many rubies—areeasiest to see when light reflects from their surfaces. But you can’t usevertical overhead illumination to see internal characteristics like thesebecause reflections from the gem’s surface block your view of thestone’s interior. Instead, you need to use a light source—such as a fiber-optic light—to direct a narrow beam of light at the stone from anoblique angle. Light entering the stone from that angle reflects frominternal fractures, cleavages, and fingerprints, and makes them mucheasier to see.

DIFFUSED LIGHTING

For diffused lighting, open the baffle and the iris diaphragm and coverthe stage opening with a white, translucent material. You can use facialtissue or even the white plastic diffuser from the microscope’s overheadlight source. Diffused light can help you detect liquid inclusions in naturalgems. It’s especially good for detecting curved color banding in flame-fusion synthetics. And it’s excellent for detecting uneven color zoning indiffusion-treated corundum, where surface-related color often stands outagainst the white background.


22


You can create diffused light by open-ing the baffle and placing a tissue or apiece of translucent white plastic on thestage over the well (top). Diffused light-ing reveals uneven color zoning in thisdiffusion-treated sapphire (bottom).

Eric Welch/GIA

To create vertical overhead illumination with a stone like this coral cabochon, anglethe stone so light strikes at about a 90º angle to its surface (above). Reflected over-head light and magnification reveal the characteristic straight, irregular, fibrousstructure in this shell cameo (left).

POLARIZED LIGHTING

You can create polarized light by opening the microscope’s baffle andplacing one polarizing filter over the light well and another between thestone and the objectives. You can hold the second filter or attach it to theobjectives. Your microscope then functions as a magnifying polariscope.

Use this type of lighting to distinguish included crystals from similar-looking gas bubbles or gas-filled cavities. Crystals might show interferencecolors and are often surrounded by halos caused by strain, while cavitiesor gas bubbles won’t have these features.

MAGNIFICATION

23

Eric Welch/GIA

You can create polarized light with a microscope by opening the baffle, placing onepolarizing filter over the light well, and holding another between the stone and theobjectives. Rotate the handheld polarizing filter to cross the filters.

John Koivula/GIA

By revealing interference colors, polarized light can help you distinguishbetween included crystals and gas bubbles or gas-filled cavities, whichshow no color.

Liquid inclusions, curved color banding,and uneven color zoning show up best indiffused lighting.

EXAMINING A STONE

As you become more comfortable with the microscope, the steps involvedin examining a stone will become second nature to you. The first step isalways to thoroughly clean the stone you’re about to examine. This isvery important: It’s easy to mistake grease and dust on the gem’s surfacefor abrasions or even inclusions.

It’s important to hold the stone steady. If you use a stoneholder, attach itto the stage and position the stone over the light well. If you hold the stonein tweezers, rest them gently on the side of the light well.

Examine the stone thoroughly. If the gem is transparent to translucent,examine both its surface and its interior. As you examine the stone, recordwhat you see on the Gem Identification worksheet. If possible, turn offthe other lights in your area while you’re working.

1. Set the magnification to 10X. Always start at this magnificationlevel.

2. Start with the well light turned off, and use the microscope’s overheadlight to examine the gem’s surface. Position the light and hold thestone so light reflects from its facets. Look at the top and the bottom,then all the way around the sides.

3. If your stone is transparent to translucent, examine its interior next.Turn off the overhead light and turn on the microscope’s internal lightsource. Make sure the baffle is closed and the iris diaphragm is com-pletely open.


24


To examine the surface of a stone, turn the well light off and use the microscope’s overhead light. Rotate the stone to examineevery side.

Always start the examination process at10X magnification.

4. To examine the interior, start by focusing on the surface, then holdthe stone still and move the focus down slowly until the back of thegem comes into focus. Slowly raise the focus back to the gem’supper surface.

5. Repeat the process from the top, from the bottom, and from every sideto make sure you view the interior of the stone from every possibleviewing angle.

6. Switch to higher-power magnification to identify any characteristics youcan’t see at 10X. This will also help you determine the nature of hard-to-see characteristics. If you have a microscope with a zoom system,you’ll soon learn to move from low to high magnification with ease.

MAGNIFICATION

25


To examine a stone’s interior, turn the well light on, close the baffle, and turn theoverhead light off. Vary the focus between the stone’s upper and lower surfaces to thoroughly examine its interior. Turn the stone to several positions and repeat the process.


26

The splitting of light in a doubly refractive (DR) gem producesdoubling: doubled images of facet junctions, inclusions, and othercharacteristics. To see doubling, you must look at the object throughthe stone—but not in an optic axis direction.

Although birefringence, which is the cause of doubling, is a veryconstant property, the amount of doubling you see varies with thestone’s size. The larger the stone, the more doubling you’ll see. Italso depends on your viewing angle when you observe the stone.

Doubling is hard to see in gems like quartz and corundum, butreadily visible in calcite, synthetic moissanite, and synthetic rutile.It’s especially useful for proving that over-the-limits stones likezircon are DR. With practice, you can even estimate birefringenceby judging the separation between the doubled images.

When you look for doubling:

• Always use the same power, such as 10X or 20X.

• Look through the stone to the opposite side. Look for doubledimages of facet junctions, inclusions, and scratches. Make sure that an image isn’t just a reflection. (This can occur close to facetjunctions.)

• Look in at least three different directions to make sure you’re notlooking down an optic axis. The strength of the doubling alsovaries with direction. Estimate birefringence in the direction ofgreatest doubling.

• To confirm doubling with a microscope, hold a polarizing filterbetween the stone and the microscope’s objectives. When yourotate the filter back and forth about 90º, the doubling appears anddisappears.

Seeing Doubling

All by Terri Weimer/GIA

If you look through a DR stone in an optic axis direction, you won’t see doubling(left). As you rotate the stone farther away from the optic axis, the doubling keepsincreasing (center and right).

MAGNIFICATION

27

Both by Terri Weimer/GIA

You might see doubling in some transparent DR stones when you examine them in darkfield lighting (left). You can confirm doubling by holding a polarizer betweenthe stone and the objectives and rotating it, making the doubling appear and disappear (right).

Taijin Lu/GIA

When you look at a synthetic moissanite under magnification, you see doubleimages of its facet junctions (20X).

Low power is best for detecting overall patterns of clarity character-istics, while high power is best for identifying and examining individualcharacteristics in detail. Use only as much magnification as you need toidentify an inclusion. If you magnify the feature too much, you mightmiss important patterns that would be more obvious at lower power.

Now that you’ve been through the basic steps of examining gemswith both the loupe and the microscope, it’s time to identify the basiccharacteristics you’re looking for in natural and synthetic gemstones aswell as imitations.

IDENTIFYING CLARITY CHARACTERISTICSn Which inclusions are typical of natural gems?

n Which characteristics are typical of synthetic and imitation gems?

n How can the microscope help you identify assembledstones?

Clarity characteristics are often the key to a gem’s identity. They canhelp separate a gem that grew naturally in the earth from one grownsynthetically in a laboratory. In rare cases, inclusions can even helpestablish the geographic origin of an important natural ruby, sapphire,or emerald. They can also usually tell you if a gem has been treated.

Later, in the separation assignments, you’ll read much more about theinclusions that identify specific gems. This section is a brief overview toremind you of the various types of characteristics you’ll encounter,starting at the gem’s surface.


28

Both by Nicholas DelRe/GIA

Low magnification is best for detecting overall patterns of clarity characteristics(above), while high magnification is best for close examination of individual characteristics (left).

SURFACE CHARACTERISTICS

As you’ve learned, you use overhead, reflected light to examine a stone’ssurface. Pay attention to areas that are vulnerable to damage, like thegirdle and culet.

You can hold large stones with tweezers or your fingers. Hold smallstones with tweezers. With a loupe, look for blemishes like scratchesand abrasions that indicate a stone’s hardness. Polishing wheels tend to

MAGNIFICATION

29

Alan Jobbins

Gems that grow in the earth often contain clarity characteristics—such as mineralcrystals and needles—that you won’t see in synthetic stones. This helps you separatenatural gems from synthetics.

John Koivula/GIA

A gold or platinum platelet is a telltale sign of a hydrothermal or flux synthetic gem.The environments these synthetics grow in often contain those metals, and micro-scopic remnants end up in the stones.

round the facet edges of gems with hardness below Mohs 7. If a gem’shardness is below Mohs 8 and it has been worn for any length of time,its facet edges will probably be abraded.

Heat treatment leaves gems like colorless or blue zircon and tanzaniteslightly brittle, so these gems often have abraded facet junctions. Heat-treated corundum often has melted surfaces along the girdle and inother areas.

Glass and plastic imitations are often shaped in molds, resulting inrounded facet junctions. As molten glass cools, it shrinks slightly, oftenproducing concave facets with surface texture that gemologists call the“orange-peel” effect.

Fractures that reach the surface can sometimes help you identify a gem.The surfaces of fractures in aggregates often have a granular, rough, or


30

Maha Tannous/GIA

Characteristics like melted surfaces are evidence of heat treatment in corundum.

Terri Weimer/GIA

Rounded facet junctions and the orange-peel effect are signs of molded glass.

The amount of wear a gemstone showscan indicate its hardness.

irregular texture, like the surface of a sugar cube. Fibrous materials liketiger’s-eye quartz can have splintery fracture surfaces.

Most transparent gems, such as quartz, beryl, corundum, and tour-maline, have a conchoidal, or shell-like, fracture surface. If the stonehas cleavage, you might see either flat cleavage faces or small con-choidal fractures alternating with flat cleavages, creating a step-likeappearance.

Surface-reaching fractures can contain some of the oil or dye usedto conceal the fractures or to enhance the gem’s color. You can locatethem by examining the gem’s surface in reflected light. Once you’vefound a surface-reaching fracture, switch to darkfield illumination tofollow the fracture as it extends into the gem and look for signs of afilling material.

MAGNIFICATION

31

John Koivula/GIA

Reflected light is best for finding surface-reaching fractures like this one. It’s beenfilled, which makes its length difficult todetermine. Switching to darkfield wouldhelp you detect the filling.

There are three methods you can use to distinguish between a trans-parent gem’s external and internal characteristics. With a microscope,it’s best to use fairly high magnification—30X to 50X.

REFLECTED LIGHT

Reflected light is the best and most widely used method. Hold thestone so light reflects from the surface the object appears to be on. Ifthe object is external, such as a piece of dust on a facet, it will standout. If it’s internal, you’ll see an unbroken, mirror-like reflection fromthe facet.

PLANE OF FOCUS

Hold the stone so you’re looking at the surface the object appears tobe on. Try to bring both the surface and the object into the sharpestpossible focus. If both come into focus at the same time, the object ison the surface or extremely close to it. If the surface comes into focusfirst, the object is inside the gem.

ROCKING

Choose a nearby feature you know is external—such as a facetjunction or a scratch—to use as a reference. Then rock the stoneslowly back and forth while you watch the object in question and thereference feature. If the object is on the same surface as the reference,it will move the same amount. If it’s within the gem, the object willmove less.

Is the Characteristic Internal or External?

Peter Johnston/GIA

When you use the plane-of-focus tech-nique, you know an object is on the facetsurface when the object and the surfaceare in focus at the same time (top). If theobject is below the surface, it’s out offocus when the facet surface is in focus(center), and in focus when the facet sur-face is out of focus (bottom).

INCLUSIONS IN NATURAL GEMS

Natural gems often have an abundance of clarity characteristics. Ingeneral, natural gems contain a far greater range of characteristics thansynthetic ones.

Natural gems grow in an environment—the earth’s rocks—wherethey’re in competition with many other minerals for the ingredients ofcrystal growth. As they grow, they often trap other minerals as includedcrystals. By comparison, synthetic and imitation stones grow in muchmore controlled surroundings—the pristine environment of the laboratoryor factory. As a result, there’s less opportunity for them to acquire foreignmaterials as they grow.

This means that when you see a range of different mineral inclusions,you know you’re looking at a natural gem. A natural ruby, for example,might contain a variety of included mineral crystals. You might seecolorless, rounded calcite, zircon, or apatite crystals and dense patternsof tiny, slender rutile needles that intersect to form silk.

Needles can be solid or hollow. If they’re hollow, they might be filledwith liquid or gas. Solid needles occur in corundum, garnet, and someemeralds. Hollow needles are frequent features of chrysoberyl. In tour-maline and beryl, the hollow needles are called growth tubes. They’reoften much coarser than hollow needles in other gems, and might becapped by tiny included crystals.

Gems that grow in mineral-rich, watery solutions often contain liquidinclusions. Topaz, beryl, and quartz can have abundant liquid inclusions.Sometimes an inclusion also contains a gas or a solid, or both. Whenonly two of those things are present—a liquid and, typically, a gas—it’s atwo-phase inclusion. If all three are present, it’s a three-phase inclusion.


32

Needle—A long, thin inclusion thatcan be a solid crystal or a hollowtube that might be filled with liquidor gas.

Silk—Group of fine, needle-likeinclusions.

Two-phase inclusion—A cavity ina gem filled with a liquid and,typically, a gas.

Three-phase inclusion—A cavityin a gem filled with a liquid, a gas,and one or more crystals.

Edward Gübelin/GIA

Needle-like mineral inclusions in emerald (above) and ruby (left) mean these gemsare natural. The group of intersecting needles in the ruby is called silk.

Kari A. Kinnunen

Blocked crystal growth causes hollowor filled growth tubes in beryl.

Sometimes, a gem might contain angular spaces that adopted theshape and symmetry of the enclosing gem crystal when it cooled. Theylook like mineral inclusions, but they’re not. These hollow areas arecalled negative crystals, and they usually contain a liquid or a gas, orboth.

Negative crystals are common in corundum, quartz, topaz, and beryl.They can also occur in synthetic gems, so you’ll have to look for otherevidence to be sure the gem is natural. If you suspect you’re looking ata negative crystal, you can use polarized light to confirm it. Unlike asolid mineral crystal, a negative crystal shows no strain colors.

MAGNIFICATION

33

Negative crystal—An angular, hollow space within a gem thatresembles a mineral inclusion.

John Koivula/GIA

Three-phase inclusions are evidence of naturally formed emeralds. They contain aliquid, a solid, and a gas.

Eduard Gübelin/GIA

Negative crystals are angular, hollow spaces that usually contain a liquid and a gas.

Natural gems typically contain a fargreater variety of inclusions than synthetic gems.

Crystals often fracture during growth. Sometimes fluids seep into thebreaks and become trapped as the fracture recrystallizes. If a breakdoesn’t heal completely, it creates a pattern of small, disconnected fluidinclusions within the stone. Because of its appearance, the inclusion iscalled a fingerprint. Fingerprints can also consist of included crystals,two-phase or three-phase inclusions, or negative crystals, as long asthey form a fingerprint-like pattern.

Inclusions can be so tiny and numerous that it’s hard to see themindividually, even at the highest magnification. When they’re numerousenough, describe them as a cloud. A cloud is any hazy or milky areathat can’t be described as a feather, fingerprint, or group of includedcrystals or needles. Many diamonds, rubies, and sapphires containclouds.

As crystals grow, their growth stages often show up as color zoning.Color zoning is a pattern of alternating light and dark areas or ofdifferent colors. It’s often seen in gems like corundum, quartz, andtourmaline. It’s caused by variations in trace elements during crystalgrowth.

In natural gems, the bands are straight and angular, following thegem’s crystal structure. Synthetics can have straight or angular colorzoning, which indicate flux and hydrothermal growth processes, or


34

Michael Waitzman/GIA

Fingerprint inclusions can contain a variety of materials. The fingerprint in the spinel(above) contains crystals and negative crystals. The fingerprint in the first blue sap-phire (top left) contains negative crystals and two-phase or liquid inclusions. Theother blue sapphire (bottom left) has a fingerprint that’s composed of liquid withinpartially healed fractures.

Both by Mike Havstad/GIA

Cloud—Any hazy or milky areathat cannot be described as afeather, fingerprint, or group ofincluded crystals or needles.

Fingerprint—Inclusions that form apattern that often resembles ahuman fingerprint.

Robert Kane/GIA

Distinctly bluish clouds often occur inVietnamese rubies.

curved color zoning, which means they were produced by flame-fusionor pulling processes.

INCLUSIONS IN SYNTHETICS AND IMITATIONS

The inclusions in synthetic gems often indicate the growth processthe manufacturer used to produce them. You might see gas bubbles insynthetics produced by the flame-fusion or pulling processes. They’reespecially likely in flame-fusion synthetics, where they can be spherical,elongated, or distorted. Spherical gas bubbles might have dark centersthat make them look like doughnuts.

The only untreated natural materials that contain gas bubbles arenatural glasses like obsidian and moldavite and natural resins like amber.Gas bubbles occur in these natural amorphous materials, but almostnever in natural crystalline materials, except as part of two-phase orthree-phase inclusions or in the junction planes of assembled stones.They might also occur in glass or plastic fillers or where mineral inclu-sions were melted by heat treatment. Some natural gems can containrounded crystals that resemble gas bubbles.

Synthetics grown by the flux process usually contain inclusions thatare remnants of the medium that the ingredients for crystal growth weredissolved in. Although they’re often thick and coarse looking, resembling

MAGNIFICATION

35

Rolf Schwieger

Angular color zoning (above) and straight color zoning (right) often occur in bluesapphires. The zoning follows the gem’s crystal structure.

John Koivula/GIA

John Koivula/GIA

Gas bubbles are common in flame-fusion synthetics like this manmade ruby.

The only untreated natural gem materialsthat contain gas bubbles are naturalglasses and resins.

icicles, they can also be delicate in appearance. Gemologists oftendescribe the delicate ones as “wispy veils.” They can resemble thefingerprints in natural gems, but are often folded and twisted, whilefingerprints usually look flattened. Flux has higher relief than finger-prints, and it might contain trapped gas bubbles. It’s normally white,but it can be colorless, yellow, orange, or brown.

As you read earlier, flame-fusion synthetics often show curved growth.Flame-fusion synthetic blue sapphires might show curved color banding.Unlike curved striae, the bands are different tones of the same color—often with alternating colorless bands—but they’re still curved. You’llnever see curved striae and curved color banding in natural crystals.

Flux and hydrothermal synthetics can show straight or angular colorzoning like that seen in natural material. The difference is that the zoning isgenerally more uniform in the synthetics than it is in natural stones. This is


36

John Koivula/GIA

The thick, coarse, grainy texture of this inclusion is typical ofmaterial trapped in synthetic ruby crystals as they grow by theflux process.

Duncan Pay

The appearance of included flux can vary according to themanufacturer’s process. These yellow-to-orange flux inclusionsappear in a Ramaura synthetic ruby.

Karl Schmetzer

Some flux inclusions look delicate and are called wispy veils.These appear in a Russian synthetic alexandrite.

John Koivula/GIA

These large, white flux inclusions in a synthetic emerald areinterconnected with smaller flux channels.

John Koivula/GIA

Flame-fusion synthetic blue sapphireslike this often show curved color band-ing. Here, it’s in the gem’s crown.

because of the more controlled environment the synthetics grow in.Some hydrothermal synthetic emeralds contain nailhead spicules, whichare cone-shaped spaces extending from synthetic crystal inclusions.

Modern heat-treatment techniques can alter many natural corunduminclusions so much that it can be difficult to separate them from someflux-grown synthetics. You’ll learn much more about the inclusions insynthetic stones in Assignment 11.

IDENTIFYING ASSEMBLED STONES

You’ve learned that assembled stones are composed of two or more piecesof material glued or fused together to form one piece. In Assignment 2,you learned some ways to identify them. Typically, a 10X loupe is usefulfor detecting signs of assembly. It’s important to be careful, however, not

MAGNIFICATION

37

Some hydrothermal synthetic emeralds can show liquid and two-phase inclusionsand fingerprint-like patterns.

John Koivula/GIA

In some hydrothermal synthetic emeralds, growth blockage can cause nailheadspicules to form. They’re usually near the seed plate and point away from it in thegrowth direction.

Flame-fusion synthetics might showcurved striae or curved color banding, butnatural gems never do.

to confuse the multiple girdle planes that can occur in natural gems withthe separation planes that indicate the presence of assembled parts.

The ability to examine a gemstone under magnification is an absolutelyindispensable skill for the gemologist. Now that you understand thebasics, you should get as much practice as possible. In later assignments,as you test stones in your practice sets, take the time to look at each ofthem carefully. The gems in your practice sets can teach you a lot. Inthe future, something you see in these sets might help you identify adifficult gem.

Even if you have access to a microscope, don’t rely on it exclusively.Practice with a loupe as well, because on buying trips, you’ll usuallyhave to depend on it. In the next assignment, you’ll learn about thespectroscope. It’s another compact, portable instrument that you mightwant to take along with you.


38

When inclusions that show in only natural or synthetic stonesappear together in the same stone, you know it’s assembled.This sapphire-synthetic sapphire doublet shows straightgrowth and gas bubbles.

Rene Moore/GIA

Heat treatment damaged this sapphire’s surface, forcing thecutter to repolish the gem. Repolishing added extra girdleplanes. Don’t mistake them for the separation planes that indi-cate an assembled stone.

Curved color banding and a separation plane between the green crown and bluepavilion prove this is an assembled stone.

MAGNIFICATION

39

Magnification is a valuable tool for detecting treatments andfor separating natural gems from their synthetic counterparts.

Practical experience and up-to-date knowledge are the keys to using magnification successfully in gem identification.

A fully corrected 10X triplet loupe is an essential gemologicaltool.

Reflected light is best for examining a stone’s surface.

Darkfield illumination works best for examining the interiors of transparent stones.

Use brightfield illumination to detect low-relief features likecurved striae.

Liquid inclusions, curved color banding, and uneven color zoning show up best in diffused lighting.

Always start the examination process at 10X magnification.

The amount of wear a gemstone shows can indicate its hardness.

Natural gems typically contain a far greater variety of inclusions than synthetic gems.

The only untreated natural gem materials that contain gasbubbles are natural glasses and resins.

Flame-fusion synthetics might show curved striae or curvedcolor banding, but natural gems never do.


40

Baffle—A small metal plate that you can close toprevent the microscope’s light from shining directlythrough the stone from below.

Base—The support platform that contains a micro-scope’s electrical controls.

Blemish—Characteristic or irregularity confined to the surface of a polished gemstone.

Chromatic aberration—Color distortion caused by the inability of a lens to bring the various coloredwavelengths of light into focus at the same distance.

Cloud—Any hazy or milky area that cannot bedescribed as a feather, fingerprint, or group of included crystals or needles.

Curved striae—Curved growth patterns seen inflame-fusion synthetics.

Darkfield illumination—Lighting of a gemstone fromthe side against a black, non-reflective background.

Depth of field—The distance that’s clear and sharp in front of and behind the point you focus on.

Fingerprint—Inclusions that form a pattern that oftenresembles a human fingerprint.

Focal distance—The distance from the surface of alens to a point that’s in sharp focus.

Fully corrected triplet loupe—A loupe that contains a three-part lens that magnifies and corrects forspherical and chromatic aberration.

Included crystal—A mineral crystal trapped within agem as it grows.

Inclusion—A characteristic enclosed within a gem-stone or reaching its surface from the interior.

Iris diaphragm—A device in the microscope’s stagethat you can open or close to control the amount oflight coming from the light well.

Light well—Housing for a microscope’s light source,located below the stage.

Liquid inclusion—Pocket in a gem that’s filled withfluids and, sometimes, gas bubbles and crystals.

Needle—A long, thin inclusion that can be a solidcrystal or a hollow tube that might be filled with liquid or gas.

Negative crystal—An angular, hollow space within a gem that resembles a mineral inclusion.

Objectives—The lenses nearest the stone on agemological microscope.

Oculars—The eyepiece lenses on a gemologicalmicroscope.

Pod—The housing for a microscope’s optical system,also called the head.

Reflected lighting—Illumination of a gem’s surface by reflecting light from it.

Relief—Contrast between an inclusion and its hostgem.

Silk—Group of fine, needle-like inclusions.

Spherical aberration—Blurring around the edges that occurs when a lens can’t get an entire image in focus at the same time.

Stage—A microscope’s working platform.

Stoneholder—A device that attaches to a micro-scope’s stage to hold a gem steady.

Three-phase inclusion—A cavity in a gem filled witha liquid, a gas, and one or more crystals.

Two-phase inclusion—A cavity in a gem filled with a liquid and, typically, a gas.

key terms

MAGNIFICATION

41

ASSIGNMENT 6

QUESTIONNAIRE

Each of the questions or incomplete statements below is followed by several possible answers. Choosethe ONE that BEST answers the question or completes the statement. Then place the letter (A, B, C, orD) corresponding to your answer in the blank at the left of the question.

If you’re unsure about any question, go back, review the assignment, and find the correct answer.When you’ve answered all the questions, transfer your answers to the answer sheet.

________1. Diffused lighting is most effective for detecting

A. thin, flat inclusions.B. high-relief inclusions.C. surface features like glass-filled cavities.D. curved color banding in flame-fusion synthetics.

________2. A fringe of color that occurs when a lens focuses different wavelengths of light at different distances is a result of

A. astigmatism.B. full correction.C. spherical aberration.D. chromatic aberration.

________3. The distance from the surface of a lens to a point that’s in sharp focus is called

A. relief.B. depth of field.C. focal distance.D. spherical aberration.

________4. If a microscope’s oculars are 15X and the zoom adjustment is set at 2X, the magnification is

A. 7X.B. 15X.C. 17X.D. 30X.

IF YOU NEED HELP: Contact your instructor through GIA online, or call 800-421-7250 toll-free in the US and Canada, or 760-603-4000; afterhours you can leave a message.

CONTINUED NEXT PAGE...


42

________5. The amount of wear a gemstone shows can indicate its

A. density.B. hardness.C. optic character.D. specific gravity.

________6. Under the microscope, if the inclusion and the facet surface are both in focus at the sametime, the inclusion is probably

A. a fracture.B. high relief.C. deep inside the stone.D. very near the surface.

________7. A long, thin inclusion that can be a solid crystal or a hollow tube is called a

A. cloud.B. needle.C. fingerprint.D. negative crystal.

________8. A natural, untreated gem material that might contain gas bubbles is

A. ruby.B. peridot.C. obsidian.D. diamond.

________9. The contrast between an inclusion and its host gem is called

A. relief.B. saturation.C. fluorescence.D. distinctiveness.

________10. The small metal flap that can be closed to prevent a microscope’s light from shiningdirectly through the stone is called the

A. pod.B. baffle.C. ocular.D. light port. CONTINUED NEXT PAGE...

MAGNIFICATION

43

________11. To examine a gem’s surface, use

A. reflected lighting.B. darkfield lighting. C. monochromatic light.D. brightfield illumination.

________12. If you find curved striae or curved color banding in a gem, you know it is

A. natural.B. synthetic.C. heat treated.D. fracture filled.

________13. You can be sure that a gem is natural if it contains

A. a feather.B. wispy veils.C. straight color banding.D. a range of different mineral inclusions.

________14. Which of these would probably have concave facets?

A. Molded gemsB. Enhanced gemsC. Assembled gemsD. Synthetic materials

________15. An angular, hollow space within a gem that resembles a mineral inclusion is called a

A. cloud.B. cavity.C. feather.D. negative crystal.

CONTINUED NEXT PAGE...


44

Distance Education students: The following questions ask you to examine the stones in the set you’recurrently working with. Choose the best answer to each question and continue filling in your answersheet as you did with questions 1 through 15.

________16. Which of the following do you see in stone 4 using 10X magnification?

A. DoublingB. Wispy veilsC. Gas bubblesD. Curved striae


A. Wispy veilsB. Curved striaeC. Included crystalsD. Three-phase inclusions


A. Curved striaeB. Flux inclusionsC. Nailhead spiculesD. Natural inclusions


A. Wispy veilsB. Curved striaeC. Included crystalsD. Three-phase inclusions


A. Wispy veilsB. Negative crystalsC. Included crystals D. Curved color banding

1. Introduction

2. General Observation

3. Refraction and the Refractometer

4. Polariscope Testing

5. Pleochroism and the Dichroscope

6. Magnification

7. Selective Absorption and theSpectroscope

8. Fluorescence and Phosphorescence

9. Additional Tests

10. Separation and Identification

11. Separating Natural Gems fromSynthetics and Imitations

12. Detecting Gem Treatments

13. Separating Red, Pink, and Purple Gems

14. Separating Blue and Violet Gems

15. Separating Green Gems

16. Separating Orange, Yellow, and Brown Gems

17. Separating Colorless, White, Gray, and Black Gems

18. Identifying Rough Gems, Parcels, and Mounted Gems

19. Advanced Laboratory Testing

gem identification

Documents