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 = 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 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

corundum Various

Star sapphires

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

Jade, variety jadeite

Chrysoberyl BeAl 2 O 4 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 gallery

Amethyst Citrine

Amethyst

ametrine

Milky quartz Smokey quartz Rose quartz

Aventurine quartz Rutilated quartz

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.

Onyx Chalcedony is the fibrous form of quartz Petrified wood

Jasper

Other natural low temperature forms of SiO 2 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 SiO 2, which scatter the light to produce the colours.

Crocidolite 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 Orthoclase var. amazonite or amazonstone

Cordierite Var. iolite

iolite