Gemstones

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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 Cs 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 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

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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 BeAl2O4

• Cats 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
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Turquoise
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The garnet picture gallery
The garnet picture gallery
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Citrine
Amethyst
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Amethyst
Citrine
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ametrine
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Rose quartz
Smokey quartz
Milky quartz
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Aventurine quartz
Rutilated quartz
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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 Earths crust.
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Chalcedony is the fibrous form of quartz
Petrified wood
Onyx
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Jasper
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Other natural low temperature forms of SiO2
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 SiO2, which scatter the light to
produce the colours.
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Tiger eye
Crocidolite
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Cats eye
Tiger eye
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Interference colours in opal, cats eye, tigers
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 chatoyancy of reflected light
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Labradorite (Schiller structure)
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Green Microcline or Orthoclasevar. amazonite or
amazonstone
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Cordierite
Var. iolite
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iolite