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Gemstones

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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Gemstones

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  1. Gemstones

  2. “ A gem is a mineral which, by cutting and polishing, possesses sufficient beauty to be used in jewelry or for personal adornment”

  3. Attributes of Gemstones • Beauty colour lustre transparency – some exceptions brilliance fire • Durability toughness hardness } enhanced by cutting

  4. Attributes of Gemstones (cont.) • Rareity • Fashion • Portability : high vallue per unit of weight; easily transported

  5. Diamond

  6. 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”

  7. Diamond weight is expressed in carats. The 1877 carat = 0.2056 gm The metric carat = 0.2 gm

  8. Beryl

  9. Gemstone beryl varieties • Goshenite – colourless • Morganite – pink • Aquamarine – blue-green • Golden beryl – yellow • Emerald – deep green

  10. corundum

  11. 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

  12. corundum Various

  13. Star sapphires

  14. Jadeite, a pyroxene One form of jade. The other is Nephrite, variety of tremolite-actinolite

  15. Jade, variety jadeite

  16. Chrysoberyl BeAl2O4 • Cat’s eye – chatoyency • Alexandrite – green in daylight; red in incandescent light

  17. Tourmaline

  18. Tourmaline comes in many different colours including multicoloured varieties, e.g. watermelon tourmaline. Rubellite – red to pink Brazilian emerald – green Indicolite - blue

  19. Zircon

  20. This is a cut crystal Peridot - gem quality olivine Olivine

  21. Turquoise

  22. The garnet picture gallery The garnet picture gallery

  23. Citrine Amethyst

  24. Amethyst Citrine

  25. ametrine

  26. Rose quartz Smokey quartz Milky quartz

  27. Aventurine quartz Rutilated quartz

  28. 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.

  29. Chalcedony is the fibrous form of quartz Petrified wood Onyx

  30. Jasper

  31. Other natural low temperature forms of SiO2 2. Opal Opal is an amorphous form of silica formed from supersaturated Si-rich solutions.

  32. Where dothecolours inopalcome from? Electron micrographs showing small spheres of amorphous SiO2, which scatter the light to produce the colours.

  33. Tiger eye Crocidolite

  34. Cats eye Tiger eye

  35. 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

  36. Labradorite (Schiller structure)

  37. Green Microcline or Orthoclasevar. amazonite or amazonstone

  38. Cordierite Var. iolite

  39. iolite

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