Good quality crystals are sought after by jewelers and Tanzania's resources are almost exhausted. The crystals are formed as a result of the metamorphism of plagioclases. The colouration is due to the presence of chromium and strontium within the crystal structure. When heated, yellow and brown tints disappear and blue hues deepen.

Mohs' Hardness 6.5-7.
Specific gravity 3.11-3.40.
Crystal structure Orthorhombic.
Colour Ultramarine to sapphire blue, violet, amethyst. Range Tanzania.

This is the most highly prized form of quartz. High quality crystals are still used as a semi-precious gem. Amethyst crystals always grow from a base. The colour is due to the precence of ferric iron. The distribution of coour bands and wavy parallel lines distinguishes.

Mohs' Hardness 7.
Specific Gravity 2.63-2.65.
Crystal structure Hexagonal.
Colour Violet, Purple or pink.
Range Brazil, Australia,Canada, USA, Russia, India, South Africa, Sri Lanka.

Most commercial citrines are heattreated amethyst or smoky quartz. Citrine is widely used as an imitation of topaz. Larger citrine crystals are pristamic with pyramid ends. The crystals will turn white if heated and dark if exposed to X-rays. Cut crystals display good luster.

Mohs' Hardness 7.
Specific gravity 2.65.
Crystal Structure Hexagonal.
Colour Pure yellow to dull yellow, honey or brownish yellow.
Range Brazil,France Madagascar, USA, Russia.

The emerald has long been valued as a gemstone, with deep-gteen crystals commanding the highest prces. Emeralds contain inclusion, a sign of authanticity, and do not necessarily dectract from their value. Synthetic emeralds are now being produced with inclusion similar to those in natural crystals.

Mohs' Hardness 7.5-8.
Specific gravity 2.67-2.78.
Crystal Structure Hexagonal.
Colour Bright green, light green, yellow green or dark green.
Range Colombia, Africa, Brazil, India, Australia, Tanzania, Pakistan, Russia.

This material is not very valuable, dispite its attraciveness. It is usually cut into round polished pieces for necklaces of pendants. Quartz cat's eye is formed from fluids associated with instrusive magmatic phenomena. Often confused with chrysoberly cat's eye, material is sensitive to acids.

Mohs' Hardness 7.
Specific gravity 2.65.
Crystal structure Hexagonal.
Colour Semi-transparent but becomes greenish-grey or green when ground.
Range Burma, India, Srilanka, Germany.

Lower quality crystals are powered and used as high-quality cutting and polishing mediums. They tend to occur in dolomitic type limestones which have become marble-like rocks. Most rubies are mined from alluvial deposits. The colour is often uneven within the crystal and they have a soft, silky sheen due to the inclusion of minute rutile crystals.

Moh's Hardness 9.
Specific gravity 3.97-4.05.
Cryastal structure Hexagonal.
Colour Shades of red.
Range Burma, Thiland, Afghanistan, Australia, Brazil, Combodia, India, USA.

Spinel has been used in jewellery for centuries, set alongside diamonds centuries, set alongside diamonds and rubies. The spinel crystals form in limestones, where there are deposits of manganese or aluminium. If iron is presrent they will have red hues. Blue hues indicate the presence of chrome. Crystals can be transparent or opaque with a vitreous lustur.

Mohs' hardness 8.
Specific gravity 3.58-3.61.
Crystal structure Isometric.
Colour Pink, red, violet- red, pale lilac, blue, violet-blue and black.
Range Afghanistan, Burma.

Tourmaline is named after the Sinehalese word for hard rocks. High quality, crystals are cut into gemstones. Colour zoning is common, varying from green, at the base, to red at the apex. Zoned stones are often sliced and polished as ornamental objects.

Mohs' Hardness 7-7.5.
Specific gravity 3.02-3.26.
Crystal Structure Hexagonal.
Colour Pink, fiery red and deep green. Pale colours can be heat treated to intensify the colour.
Range Burma, Brazil, Srilanka, USA, Mazambique, Madagascar.

An Important source if zirconium, hafnium and thorium. Quality-grade crystals are cut into gemstones with green zircons being much in demand. Usually associated with acidic, igneous rocks or pegmatities. The crystal are four-sided and stubby, often perfectly transparent with good luster, but they can also be opaque and dull.

Mohs' Hardness 6.5-7.5.
Specific gravity 3.9-4.71.
Crystal structure Tetragonal.
Colour Colourless to yellow.
Range Cambodia, Norway, Srilnka, Thiland, Vietnam, Australia, Brazil, USA.