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Gemstones. “ A gem is a mineral which, by cutting and polishing, possesses sufficient beauty to be used in jewelry or for personal adornment”. Attributes of Gemstones. Beauty colour lustre transparency – some exceptions brilliance fire • Durability toughness
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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”
Attributes of Gemstones • Beauty colour lustre transparency – some exceptions brilliance fire • Durability toughness hardness } enhanced by cutting
Attributes of Gemstones (cont.) • Rareity • Fashion • Portability : high vallue per unit of weight; easily transported
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”
Diamond weight is expressed in carats. The 1877 carat = 0.2056 gm The metric carat = 0.2 gm
Gemstone beryl varieties • Goshenite – colourless • Morganite – pink • Aquamarine – blue-green • Golden beryl – yellow • Emerald – deep green
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
corundum Various
Jadeite, a pyroxene One form of jade. The other is Nephrite, variety of tremolite-actinolite
Chrysoberyl BeAl2O4 • Cat’s eye – chatoyency • Alexandrite – green in daylight; red in incandescent light
Tourmaline comes in many different colours including multicoloured varieties, e.g. watermelon tourmaline. Rubellite – red to pink Brazilian emerald – green Indicolite - blue
This is a cut crystal Peridot - gem quality olivine Olivine
The garnet picture gallery The garnet picture gallery
Citrine Amethyst
Amethyst Citrine
Rose quartz Smokey quartz Milky quartz
Aventurine quartz Rutilated quartz
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.
Chalcedony is the fibrous form of quartz Petrified wood Onyx
Other natural low temperature forms of SiO2 2. Opal Opal is an amorphous form of silica formed from supersaturated Si-rich solutions.
Where dothecolours inopalcome from? Electron micrographs showing small spheres of amorphous SiO2, which scatter the light to produce the colours.
Tiger eye Crocidolite
Cats eye Tiger eye
Interference colours in opal, cat’s eye, tiger’s eye and labradorite (Feldspar) Constructive interference occurs when N l = 2 n d sinq N = 1, 2, 3 Since spacing of spheres, fibers, and lamellae is close to wavelength of visible light: cats eye and tiger eye: get chatoyancyof reflected light
Cordierite Var. iolite