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Published byAnis Byrd Modified over 9 years ago
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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 Sapphire All are corundum Al 2 O 3 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 pyroxene One form of jade. The other is Nephrite, variety of tremolite- actinolite
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Jade, variety jadeite
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Chrysoberyl BeAl 2 O 4 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 gallery
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Amethyst Citrine
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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 SiO 2 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 SiO 2 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 SiO 2, which scatter the light to produce the colours.
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Crocidolite Tiger eye
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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 Orthoclase var. amazonite or amazonstone
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Cordierite Var. iolite
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iolite
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