2. Gemstones

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

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

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

6. Diamond

7. 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”

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

9. Beryl

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

11. corundum

12. 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

13. corundum Various

14. Star sapphires

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

16. Jade, variety jadeite

17. Chrysoberyl BeAl 2 O 4 Cat’s eye – chatoyency Alexandrite – green in daylight; red in incandescent light

18. Tourmaline

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

20. Zircon

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

22. Turquoise

23. The garnet picture gallery

24. Amethyst Citrine

25. Amethyst

26. ametrine

27. Milky quartz Smokey quartz Rose quartz

28. Aventurine quartz Rutilated quartz

29. 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.

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

31. Jasper

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

33. 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.

34. Crocidolite Tiger eye

35. Cats eye

36. 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

37. Labradorite (Schiller structure)

38. Green Microcline or Orthoclase var. amazonite or amazonstone

39. Cordierite Var. iolite

40. iolite