gemstones “ a gem is a mineral which, by cutting and polishing, possesses sufficient beauty to be...

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