Gemstones

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A presentation on the mineralogy of common gemstones.

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Gemstones

  1. 1. Shaumik Daityari GLT, 2nd year
  2. 2. GemstoneA gemstone or gem (also called a precious or semi-precious stone, a fine gem, or jewel) is a piece ofmineral, which, in cut and polished form, is used tomake jewellery or other adornments.Certain other materials (like amber) which are notminerals, used to make jewellery, are also calledgemstones.
  3. 3. ClassificationTraditional classification in the West: Precious(diamond, ruby, sapphire and emerald) and Semi-preciousChemical Composition: Carbon (Diamond),Aluminium Oxide (Ruby)Crystal System: Cubic (Diamond), Hexagonal(Tourmaline)Groups and VarietiesRefractive index, dispersion, specific gravity,hardness, cleavage, fracture, luster, pleochlroism,birefringence, inclusionsIn terms of their “water”: first second and third water
  4. 4. Cutting and PolishingA citrine preform and the finished faceted stone. Raw Gemstones
  5. 5. Cutting and PolishingThe process of cutting and polishing gems iscalled gemcutting or lapidary.Gemstone material that has not been extensivelycut and polished is referred to generally asrough. Rough material that has been lightlyhammered to knock off brittle, fractured materialis said to have been cobbed.All gems are cut and polished by progressiveabrasion using finer and finer grits of hardersubstances.Diamond and Silicon Carbide used as abrasives.
  6. 6. TreatmentGemstones are often treated toenhance the colour or clarity of thestone. Depending on the type andextent of treatment, they can affectthe value of the stone. Sometreatments are used widelybecause the resulting gem isstable, while others are notaccepted most commonly becausethe gem colour is unstable andmay revert to the original tone.
  7. 7. DiamondDiamond (from the ancient Greek –adámas "unbreakable") is an allotrope ofcarbon, where the carbon atoms arearranged in a variation of the face-centered cubic crystal structure called adiamond lattice.Excellent optical and mechanicalproperties, combined with efficientmarketing, make diamond the mostpopular gemstone.Most natural diamonds are formed at high-pressure high-temperature conditionsexisting at depths of 140 to 190 km (87 to120 mi) in the Earth mantle. Russia, Bostwana, Australia and Congo Found in U.S, South Africa,
  8. 8. DiamondColor: Typically yellow, brown or gray to colorless.Crystal habit: Octahedral Crystal system: CubicCleavage: 111 (perfect in four directions) Fracture:ConchoidalHardness: 10 Luster: AdamantineStreak:ColorlessDiaphaneity: Transparent Specific gravity: 3.52Optical properties: Isotropic Refractive index:2.418
  9. 9. Diamond Thin Section (PP) Colour: Colourless Cleavage: Perfectly Octahedral Relief: Extremely High
  10. 10. Diamonds can beidentified by their highthermal conductivity.Their high refractiveindex is also indicative.Also have fairly highdispersion.Diamonds can scratchother diamonds, but Most gemologists still rely uponthis can result in skilled use of a loupedamage to one or both (magnifying glass) to identifystones. So hardness diamonds by eye.tests rarely used.
  11. 11. CorundumCorundum is a crystalline form ofaluminium oxide with traces of iron,titanium and chromium.The name "corundum" is derivedfrom the Tamil word "kuruntam"meaning “ruby”.Mostly found in Greece and US
  12. 12. CorundumColor: Colorless, gray, brown; pink to pigeon-bloodredCrystal habit: Rhombohedral crystals, massive orgranularCrystal system: Trigonal (HexagonalScalenohedral)Cleavage: None, Rectangular Parting Hardness:9Luster: AdamantineTransparency: TranslucentSpecific Gravity: 3.9 - 4.1 Streak: White
  13. 13. Corundum: Optical PropertiesColour: Colourless, pale pink, green, blue or yellowRelief: Very HighPleochlroism: YesUniaxial (negative)Extinction: ParallelWeakly BirefringentTwinning: Yes
  14. 14. Corundum in PP Light
  15. 15. RubyA ruby is a pink to blood-redcoloured gemstone, a varietyof the mineral corundum.Its name comes from ruber,Latin for red.The brightest and mostvaluable "red" called pigeonblood-red, commands a hugepremium over other rubies ofsimilar quality.Found in Myanmar, Thailand,India, Pakistan, Afghanistan,Sri Lanka, Madagascar,Nepal and recently
  16. 16. SapphireSapphire is a gemstone variety of themineral corundum, an aluminiumoxide, when it is a colour other thanred or dark pink.Trace amounts of other elements suchas iron, titanium, or chromium can givecorundum blue, yellow, pink, purple,orange, or greenish color.Name derived from Greek sappheiros,meaning "blue stone“.Found in Australia, Thailand, SriLanka, Madagascar and US.
  17. 17. Pink, Padmaraga and Star Sapphires
  18. 18. EmeraldEmerald is a variety of the mineralberyl coloured green by traceamounts of chromium andsometimes vanadium.The word "Emerald" is derived fromeither the Hebrew word izmargadmeaning "green“.Found in Afghanistan, Australia,Austria, Brazil, Bulgaria, Cambodia,Canada, China, Egypt, Ethiopia,France, Germany, India, Italy,Kazakhstan, Madagascar,Mozambique, Namibia, Nigeria,Norway, Pakistan, Russia, Somalia,South Africa, Spain, Switzerland,Tanzania, United States, Zambiaand Zimbabwe.
  19. 19. Emerald (Beryl)Colour: Blue-greenStreak: White Hardness: 7.5 – 8Cleavage: Very Poor Fracture: Even Luster:VitreousSpecific Gravity: 2.7 - 2.9 Transparency:TranslucentCrystal System: HexagonalCrystal Habit: Hexagonal Prisms
  20. 20. Emerald: Optical PropertiesColour: ColourlessRelief: Low to ModeratePleochlroism: YesUniaxial (negative)Birefringence: Low, first order graysCleavage: At right angles
  21. 21. The Gachala Emerald
  22. 22. Tourmaline Tourmaline is a crystal boron silicate mineral compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is classified as a semi-precious stone and the gem comes in a wide variety of colors. The name comes from the Sinhalese word "Thuramali“, or "Thoramalli“, which applied to different gemstones found in Sri Lanka.Found three most US, Czech Republic, The in Sri Lanka, well-known Pink Tourmaline (Elbaite) in membersAfrica Elbaite, Schorl, Brazil, are and Afghanistan Quartz
  23. 23. TourmalineColor: Black, brown, green, red, pink, blue, andgray.Streak: WhiteLuster: ResinousTransparency: Transparent to Opaque; usuallyTranslucentHardness: 7 - 7.5Crystal System: HexagonalHabit: Deformed Triangular PrismsCleavage/Fracture: Poor/Conchoidal to Uneven
  24. 24. Tourmaline: Optical PropertiesColour: ColourlessPleochlorism: YesRelief: HighUniaxial (Negative)Cleavage: None but prominent cracks visibleParallel extinctionLower 2nd order to lower 3rd order interferencecolours.
  25. 25. Tourmaline in PP and XP light
  26. 26. Thermochromism: Thecolour of some Tourmalinecan be enhanced throughheat treatment. Somegreenish stones can bemade deep green, somebrownish-red stones can bemade red, and some lightpink stones can be madecolourless through heating.Tourmaline can form inextremely aesthetic slendercrystals that are highlyvalued by collectors.Chatoyant variety used ascat’s eye.
  27. 27. QuartzQuartz is the second-most-abundant mineral in the Earthscontinental crust, after feldspar.There are many differentvarieties of quartz, several ofwhich are semi-preciousgemstones. Especially inEurope and the Middle East,varieties of quartz have beensince antiquity the mostcommonly used minerals in themaking of jewellery andhardstone carvings.Gemstone variety mostly foundin US and Brazil.
  28. 28. QuartzColour: Colourless, Milky, Purple, Pink, Yellow,BrownStreak: White Specific Gravity: 2.65 Hardness: 7Cleavage/Fracture: None/Conchoidal or BrittleLuster: Vitreous Transparency: Transparent toOpaqueCrystal Shape: HexagonalCrystal Habit: Prismatic/Massive
  29. 29. Quartz- Optical PropertiesColour: ColourlessCleavage: NoneRelief: LowUniaxial (positive)Birefringence: Low with first order greys and whites
  30. 30. Quartz in PP and XP Light
  31. 31. Onyx and Agate
  32. 32. Tiger’s EyeTigers eye is a chatoyantgemstone that is usuallya metamorphic rock thatis a golden to red-browncolour, with a silky luster.
  33. 33. Egg shape tigers eye with iron stripes
  34. 34. OlivineThe mineral olivine (when gem-quality also called peridot) is amagnesium iron silicate. It is acommon mineral in the Earthssubsurface but weathers quickly onthe surface.Found in Pakistan, Nepal,Myanmar, Russia, Italy and Norway
  35. 35. OlivineColour: Colourless, Olive Green, Greenish Yellow,GreyStreak: White Specific Gravity: 3.1 -3.4 Hardness:5.5 - 6.5Cleavage/Fracture: Poor/ConchoidalLuster: Vitreous Transparency: TranslucentCrystal Shape: OrthorhombicCrystal Habit: Prisms and Dipyramids
  36. 36. Olivine - Optical PropertiesColour: Colourless to faint greenCleavage: NoneRelief: HighBiaxialBirefringence: High with mostly third order colours
  37. 37. Olivine in PP and XP light
  38. 38. OlivineOlivine is named for itstypically olive-greencolour (thought to be aresult of traces of nickel),though it may alter to areddish colour from theoxidation of iron.Extraterrestrialoccurrence
  39. 39. ZirconZircon is a mineral belonging to the group ofnesosilicates. Its chemical name is zirconium silicate.The natural colour of zircon varies between colourless,yellow-golden, red, brown, blue, and green. Colourlessspecimens that show gem quality are a popular substitutefor diamond; these specimens are also known as "Maturadiamond".
  40. 40. ZirconColour: Colourless, Brown, Green, Grey, RedStreak: Colourless - White Specific Gravity: 4.68Hardness: 7.5 Cleavage/Fracture:Poor/ConchoidalLuster: AdamantineTransparency: Transparent- TranslucentCrystal Shape: TetragonalCrystal Habit: Square Prisms, Pyramids or acombination of the two
  41. 41. Zircon - Optical PropertiesColour: Colourless to faint greenCleavage: ImperfectRelief: Very HighBirefringence: High with mostly third to fouth ordercolours in Prismatic SectionsUniaxial (Positive)Basal Sections are isotropic
  42. 42. Zircon in PP and XP Light
  43. 43. Imitations and Artificial GemstonesSome gemstones are manufactured to imitate other gemstones.They are called imitations.The imitations copy the look and colour of the real stone butpossess neither their chemical nor physical characteristics.Lab created gemstones are not imitations. Synthetic (labcreated) corundums, including ruby and sapphire, are verycommon and they cost only a fraction of the natural stones.Whether a gemstone is a natural stone or a lab-created(synthetic) stone, the characteristics of each are the same.Lab-created stones tend to have a more vivid colour to them, asimpurities are not present in a lab and do not modify the clarityor colour of the stone.
  44. 44. Thank you!

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