gemstones “ a gem is a mineral which, by cutting and polishing, possesses sufficient beauty to be...
TRANSCRIPT
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Gemstones
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“ A gem is a mineral which, by cutting and polishing, possesses sufficient beauty to be used in jewelry or for personal adornment”
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Attributes of Gemstones
• Beauty
colour
lustre
transparency – some exceptions
brilliance
fire
• Durability
toughness
hardness
} enhanced by cutting
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Attributes of Gemstones (cont.)
• Rareity
• Fashion
• Portability : high vallue per unit of weight;
easily transported
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Diamond
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The Four “C”s of Diamond Valuation
• Colour – usually colourless, some pink or blue varieties
• Clarity – free of inclusions and fractures
• Cut – good cut enhances value
• Carat weight – the greater the weight, in carats, the higher the value per carat
and a new fifth C
• Conflict-free – no “blood diamonds”
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Diamond weight is expressed in carats.
The 1877 carat = 0.2056 gm
The metric carat = 0.2 gm
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Beryl
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Gemstone beryl varieties
• Goshenite – colourless
• Morganite – pink
• Aquamarine – blue-green
• Golden beryl – yellow
• Emerald – deep green
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corundum
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Ruby and SapphireAll are corundum Al2O3
• Ruby – red
• Sapphire – blue
Other colours are specified.
Asterism, produced by inclusions along planes related by six-fold axis, yields
Star Sapphires
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corundum
Various
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Star sapphires
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Jadeite, a pyroxeneOne form of jade. The other is
Nephrite, variety of tremolite-actinolite
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Jade, variety jadeite
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Chrysoberyl BeAl2O4
• Cat’s eye – chatoyency
• Alexandrite – green in daylight;
red in incandescent light
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Tourmaline
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Tourmaline comes in many different colours including multicoloured varieties, e.g. watermelon tourmaline.
Rubellite – red to pink
Brazilian emerald – green
Indicolite - blue
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Zircon
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Olivine
Peridot - gem quality olivine
This is a cut crystal
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Turquoise
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The garnet picture galleryThe garnet picture gallery
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Amethyst
Citrine
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Citrine
Amethyst
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ametrine
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Milky quartz
Smokey quartz
Rose quartz
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Aventurine quartz
Rutilated quartz
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Other natural low temperature forms of SiO2
1. Agate
Agate is made from very fine fibrous crystals of quartz. Agate grows from Si-rich solutions in the shallow Earth’s crust.
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Onyx
Chalcedony is the fibrous form of quartz
Petrified wood
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Jasper
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Other natural low temperature forms of SiO2
2. Opal
Opal is an amorphous form of silica formed from supersaturated Si-rich solutions.
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Where do the colours in opal come from?
Electron micrographs showing small spheres of amorphous SiO2, which scatter the light to produce the colours.
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Crocidolite
Tiger eye
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Tiger eye
Cats eye
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Interference colours in opal, cat’s eye, tiger’s eye and labradorite (Feldspar)
N = 2 n d sin
Constructive interference occurs when
Since spacing of spheres, fibers, and lamellae is close to
wavelength of visible light: cats eye and
tiger eye: get chatoyancy of reflected light
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Labradorite (Schiller structure)
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Green Microcline or Orthoclasevar. amazonite or amazonstone
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CordieriteVar. iolite
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iolite