gemstones the fascination of colour€¦ · 1! presentation by dr. michael s. krzemnicki swiss...
TRANSCRIPT
1!
Presentation by
Dr. Michael S. Krzemnicki
Swiss Gemmological Institute SSEF
Photos © M.S. Krzemnicki, SSEF, except where indicated otherwise
Gemstones &
The Fascination of Colour
Christie’s Spring Auctions 2010 Lecture
Sunday 30th May 2010
© Swiss Gemmological Institute SSEF!
Colour...!
2!
© Swiss Gemmological Institute SSEF!
...is emotion!
© Swiss Gemmological Institute SSEF
Peridot of 210 ct
from Burma (Myanmar)
3!
© Swiss Gemmological Institute SSEF
Spinel of 110 ct
from Tajikistan
Kupferhaltige Turmaline aus Mozambique!
Paraiba oder nicht ?, das ist die Frage...
Copper-bearing Tourmaline!
© Swiss Gemmological Institute SSEF
4!
Cause of Colour in Gemstones
© Swiss Gemmological Institute SSEF
1)" Chemical colours, e.g. colouring trace elements (chemical impurity)
2) Structural colours, e.g. defects in the crystal structure
3)" Physical colours, e.g. iridescence, dispersion, diffraction
4) Colours by inclusions
5) Organic colours, e.g. organic pigments
Colour Effects in Gemstones
© Swiss Gemmological Institute SSEF
1) ! Colour zoning
Due to variable uptake of colouring elements during crystal growth.
Colour zoning in Tourmaline
5!
Colour Effects in Gemstones
© Swiss Gemmological Institute SSEF
2) ! Pleochroism
Colour perception varies in different viewing directions
Blue dumortierite from Tanzania
Colour Effects in Gemstones
3) ! Colour change effect (also „alexandrite effect)
Different colour perception when viewed under different lighting sources (e.g. greenish in daylight and purple under an incandenscent lamp).
Alexandrite is the most known gemstone showing this effect, but other colour changing stones exist.
© Swiss Gemmological Institute SSEF
6!
Colour Effects in Gemstones
© Swiss Gemmological Institute SSEF
4) ! Colour „change“ when viewed in transmission or reflection
Opal
Due to diffraction of light at submicroscopic dispersed impurities when viewed in front of a light source.
Colour Observation
© Swiss Gemmological Institute SSEF
1)" We can observe colour
2)" We can see what colour is absorbed
3)" We can measure colour
o - rays
7!
SSEF Photocard
© Swiss Gemmological Institute SSEF
Diamonds
© Photos Christies HK 2010 & Swiss Gemmological Institute SSEF
© G.E. Harlow, 1978!
Even a colourless diamond shows colour, the „fire“ of the diamond is due to dispersion of white light when entering the stone.
Lot 2472: 10.05 ct
8!
The blue Wittelsbach-Graff Diamond
© Swiss Gemmological Institute SSEF
This historic diamond of 35.56 ct (after recutting 31.06 ct) sold at Christie‘s auction in November 2008 in Geneva for a record breaking 24 mio US$.
Jean-Marc Lunel (Christie‘s Geneva) and Jean-Pierre Chalain (SSEF) with the blue Wittelbach
Yellow Diamonds
© Swiss Gemmological Institute SSEF
colour due to nitrogen impurities
296 ct 325‘296 ct
9!
Yellow Diamonds
© Swiss Gemmological Institute SSEF
colour due to nitrogen impurities
Less is more for commercial colourless diamonds....
D E F G H I J K L M...
More is better for fancy coloured diamonds....
© V. Pagel-Theisen!
Ruby (variety of Corundum)
© Swiss Gemmological Institute SSEF
Ruby is the red variety of corundum coloured traces of chromium.
Chemically pure corundum (Al2O3) is colourless.
Lot 2382
10!
The Graff Ruby
© Image courtesy Graff International.
Sold at Christie's in St. Moritz in February 2006 for 3.6 mio US$ ($425,000 per carat).
This record-breaking gemstone was termed the Graff ruby and exhibits the desired "pigeon's blood" red used for highly sought rubies from Burma.
It was also noted that the ruby was certified by the SSEF lab in Switzerland confirming the authenticity of the ruby with “no indications of heating“ and its Burmese origin (Mogok).
New mines
© Swiss Gemmological Institute SSEF
20.46 cts
Excellent ruby of 8 ct from Montepuez in Mozambique.
Exceptional ruby of 20.46 ct from Winza in Tanzania.
Both rubies are unheated and exhibit an impressive purity and a well saturated colour.
11!
What is ruby ?
-" ruby ?
- pink sapphire ?
- purple sapphire ?
© Swiss Gemmological Institute SSEF
Subtle colour differences for depending on amount of trace elements.
Sapphire of 196 ct from Burma
© Swiss Gemmological Institute SSEF
Blue colour due to traces of iron (Fe2+ and Fe3+)
and titanium.
12!
Sapphire from Kashmir (approx. 40 ct)
SSEF Appendix letter:
© Swiss Gemmological Institute SSEF
© SSEF Swiss Gemmological Institute
Sapphires from Kashmir contain sub-microscopic fine inclusions which scatter the incomping light. As a result, these stones may show a
highly appreciated velvety blue colour.
© H.A. Hänni, SSEF 2004
The velvety blue of Kashmir sapphires
Lot 2387: 17.45 ct
13!
Beryl Group
© Swiss Gemmological Institute SSEF
Aquamarine
of approximately 500 ct
Chemically pure beryl is colourless. Various trace elements are responsible for colours such as blue, green, yellow, red, and pink.
Emerald (green variety of beryl)
© Swiss Gemmological Institute SSEF
Very pure emerald from Colombia. No indications of clarity modification.
14!
Engraved emeralds
© Swiss Gemmological Institute SSEF
118 ct
110 ct
The Cartier Shakti Emeralds: An exceptional pair of emeralds
From the La Pita mine in Colombia, ca. 10 cm in length.
El-Itoco Emerald, 472 ct
© Swiss Gemmological Institute SSEF
15!
Absorption spectra of ruby (top), alexandrite (middle) and emerald (bottom)
Chromium as colouring element
© Swiss Gemmological Institute SSEF
All three gemstones show similar absorption spectra with characteristic chromium absorption bands.
The density of the crystal structure (ruby = dense; alexandrite = intermediate; emerald = wider) explains why they differ in colour from red to green.
Ruby
Alexandrite
Emerald
Exceptional historical spinel necklace from Tajikistan certified at SSEF
© Swiss Gemmological Institute SSEF
In ancient times, spinel was not known as a mineral and many of the large historic
„Balass rubies“ (e.g. the Black Prince „Ruby“ in the Imperial State Crown of Great Britain) were later identified as spinels.
16!
Red Spinel from Tajikistan (Pamir)
© Swiss Gemmological Institute SSEF
Pierre Lefèvre, gemmologist at SSEF with a superb spinel of 110 cts from Tajikistan.
Spinel from Madagascar
© Swiss Gemmological Institute SSEF
Colour changing spinel (84 ct) from Madagascar
17!
Inclusions in red spinel from Tanzania
© Swiss Gemmological Institute SSEF
Oriented planes of fine „dust“ precipitates in spinel from Tanzania
© H.A. Hänni, SSEF
Luminescence
spectra of spinel
© Swiss Gemmological Institute SSEF
18!
Jadeite
Chemically pure jadeite is colourless. It belongs to the pyroxene mineral group. !
The most appreciated emerald green jadeite is coloured by chromium and is found in northern Burma.!
© Gemmological Institute SSEF
Fei Cui (Jadeite Jade)
Definition of Fei Cui (Jadeite Jade) "after the Gemmological Association of Hong Kong!
Fei Cui (jadeite jade) is a granular to fibrous polycrystalline aggregate. It is composed of jadeite as major mineral. It may consist of other minor minerals such as opmhacite, kosmochlore, amphibole, and feldspar.!
Kosmochlor (Cr)!
Omphacite (Fe)! Jadeite (Na)!
© Swiss Gemmological Institute SSEF
19!
Jadeite
Reflections and light diffusion along grain boundaries of polycrystalline jadeite may result in a bright vivid colour.!
© Swiss Gemmological Institute SSEF
SEM BSE picture!
Graphite inclusions may produce greyish and black jadeite-jade!
Natural pearl harvest in Bahrain
© Swiss Gemmological Institute SSEF
Pearl dealer Mr. Abdul Razak Al Mahmood
20!
Pearl colour
© Swiss Gemmological Institute SSEF
Orient: Iridescence due to light
diffraction and interference
Bodycolour: brown, yellow, crème, orange, pink, grey, black due to organic pigments
Conch pearls (from the Queen Conch)
© Swiss Gemmological Institute SSEF, Verdana, 10pt, normal
Characteristic pink colour and flame-structure
Flame structure, due to densely interlayered aragonite fibres
© Swiss Gemmological Institute SSEF
21!
© SSEF Swiss Gemmological Institute
Pearls from the Lion paw scallop
Matombo (Uluguru Mountains)"
Morogoro Province!
© Swiss Gemmological Institute SSEF
22!
Low temperature heating (<1000 °C), e.g. Mong Hsu rubies and pink sapphires effect: colour shift, blue colour component is reduced
Ruby treatment Verdana 24pt, bold
© Swiss Gemmological Institute SSEF
Gemstone market in Ratnapura
Thank you for your attention
Swiss Gemmological Institute SSEF
website: www.ssef.ch email: [email protected]