gemstones “ a gem is a mineral which, by cutting and polishing, possesses sufficient beauty to be...
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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
hardness
} enhanced by cutting
Attributes of Gemstones (cont.)
• Rareity
• Fashion
• Portability : high vallue per unit of weight;
easily transported
Diamond
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
Beryl
Gemstone beryl varieties
• Goshenite – colourless
• Morganite – pink
• Aquamarine – blue-green
• Golden beryl – yellow
• Emerald – deep green
corundum
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
Star sapphires
Jadeite, a pyroxeneOne form of jade. The other is
Nephrite, variety of tremolite-actinolite
Jade, variety jadeite
Chrysoberyl BeAl2O4
• Cat’s eye – chatoyency
• Alexandrite – green in daylight;
red in incandescent light
Tourmaline
Tourmaline comes in many different colours including multicoloured varieties, e.g. watermelon tourmaline.
Rubellite – red to pink
Brazilian emerald – green
Indicolite - blue
Zircon
Olivine
Peridot - gem quality olivine
This is a cut crystal
Turquoise
The garnet picture galleryThe garnet picture gallery
Amethyst
Citrine
Citrine
Amethyst
ametrine
Milky quartz
Smokey quartz
Rose 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.
Onyx
Chalcedony is the fibrous form of quartz
Petrified wood
Jasper
Other natural low temperature forms of SiO2
2. Opal
Opal is an amorphous form of silica formed from supersaturated Si-rich solutions.
Where do the colours in opal come from?
Electron micrographs showing small spheres of amorphous SiO2, which scatter the light to produce the colours.
Crocidolite
Tiger eye
Tiger eye
Cats eye
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
Labradorite (Schiller structure)
Green Microcline or Orthoclasevar. amazonite or amazonstone
CordieriteVar. iolite
iolite