Quartz is a remarkable mineral. Whilst having a simple chemical composition, SiO2, and a simple framework of hexagonally arranged silica units, it forms an unprecedented range of diverse forms and varieties. It is as if the ingredients were thrown into a pot and cooked up in a multitude of interesting and unusual ways. However, closer inspection reveals three basic recipies; where the mineral forms large regular crystals; where the crystals are so small and tightly packed that they can`t be seen at all, even with a microscope; and where there is no apparent crystalline structure at all.

When crystalised as large, well-formed, crystals or crystal crusts, the specimens are often transparent and gemmy. The main variations include:

Rock Crystal - colourless and transparent. The name was first used by the Greeks, who believed that the crystals were frozen water that had been subjected to prolonged cold in the mountains. Thus, crystallos, meaning ice. The mineral is one of the commonest semi-precious stones and is widely used in jewellery.

Amethyst - a violet, transparent variety, coloured by the presence of aluminium. Also very popular in jewellery and believed by the Greeks to protect the wearer from drunkenness. Hence the name, amethisos, meaning not drunken.

Smoky Quartz - a grey variety whose colour is caused by radioactivity from nearby when the crystal was growing. For a long time it was valued less than the former two, but has recently gained popularity.

Citrine - a yellow to brown, transparent variety. It is the rarest of the crystalline quartzes and is hence the most valuable. However, it is easily forged by the heating of amethyst.

Morion - a black, non-transparent variety. In Victorian days often worn during the period of mourning.

Rose Quartz - the colour is casued by tiny inclusions of titanium. It has the frequent property of asterism when cut into a spherical shape.

Milky Quartz - a white, opaque variety, whose colour is caused by tiny gas or liquid filled bubbles.

Priasolite - an olive-green variety often forming small crystal groups.

Eisenkiesel - an old German name for quartz covered by, and usually containing inclusions of, minute crystals of Hematite, making it blood-red.

Aventurine - is a quartz with inclusions of sheet silicates. Many colours, including the common green, caused by flakes of Fuschite.

The varieties where the crystals of Quartz are microscopic are referred to as microcrystalline quartzes. The group contains some of the most beautiful minerals in nature, due to frequent coloured inclusions of other minerals. Examples include:

Chalcedony - is a variety in which the microscopic crystals are arranged as layers of thin upright fibres. The upper surface tends to be botryoidal and smooth. The material has a waxy luster, extremely small crystals and a specific gravity less than quartz. It is nearly transparent or transluscent and will fracture in a conchoidal manner.

Chrysoprase - is the most valuable and sought-after chalcedony with a uniform rich green colour. Its value was such that it is mentioned in the Bible. The colour is due to nickel.

Agate - the most colourful form of chalcedony, having concentric bands or rings. Formed in cavities by slow deposition of silica from solution, usually in volcanic areas. Petrified wood is commonly wood replaced by agate.

Carnelian - red colour and banding from inclusions of hematite. The name comes from the Latin for meat. Highly popular gemstone, especially in Roman times.

Onyx - an agate with straight, even banding. Black and white bands are common.

Sardonyx - brown chalcedony, bands coloured by inclusion of iron oxides.

Jasper - a variety of chalcedony that contains up to 20% of foreign minerals which determine the colour, texture and appearance. Uniformly coloured jasper is rare and is usually multicoloured, striped, spotted or marbled.

The varieties where crystals are not apparent, often referred to as Amorphous Silica. These are usually colloidal precipitates with residual water.

Opal - a gel precipitated from an aqueous suspension. Contains appreciable residual water. Over time opal loses water and crystallises. However the process takes millions of yeras. Opal's luster is usually glassy or pearly but may be resinous or dull. Pure opal is colourless but colour arises from impurities. Opal comes in several varieties according to colour, texture and luster.

Common Opal - a variety of opal which is white or pale in colour.

Precious Opal - a variety of common opal with a play of rainbow colours caused by banding and internal cracks and defects. The effect is known as Opalescence.

Fire Opal - a red or yellow variety with a strong play of internal colour.

Silica Glass - formed by melting of silica. Very rare because of silica's high melting point. Therefore mostly found in igneous areas including volcanic lavas. Also called Lechatelierite it occurs as tubes (Fulgurites) formed when lightning strikes quartz sand or sandstone.

Wood Opal - pertrified wood where the woody material has been replaced by opal.

Tripolite - siliceous remains of diatoms, a marine life-form surrounded by a silica skeleton. A white powder used as a polishing and abrasive material.

Moss Opal - a white variety of opal containg branching inclusions which resemble the natural form of moss.

Hyalite - a very rare variety of opal. Usually transparent and forms thin crusts on underlying matrix rock. Often resembles ice.