Opal is found in many varieties, but precious opal represents a remarkably small percentage of the total opal mined. Fine gem quality opal is more rare than rubies and emeralds. It is a more natural gem, and is a thing of beauty and obvious worth, even in its natural state.

Black Opal
Black opal is the most rare and valuable of all opal. It is solid and generally found as a bar (or bars) of various colours in a dark body (black, blue, brown or grey). Some black opals have a complete rainbow of colours while others have deep blue-green hues.

In addition, there are also semi-black opals and black crystal opals. While a true black opal displays sharp brilliant colours on a dark background, the semi-black opal shows a background or body colour of mid-grey. In contrast, the black crystal opal is translucent with no traces of black potch on it underside. Its colours are sharp and visible beneath the surface.

Few realise that 99.9 per cent of the world's supply of this radiant gem is mined at only three pinpoints on the globeÑLightning Ridge, Mintabie and Andamooka. The majority of opal found at Mintabie and Andamooka is classified as semi-black opal.

Boulder Opal
Boulder opal is also classified as solid opal. It occurs as thin veins of precious opal in the cracks and cavities of light and dark brown ironstone boulders in Queensland. The opal flowed into the cracks and fissures in the boulders in liquid form millions of years ago. With the passing of centuries, the liquid material formed into solid opal and now miners cut these stones into magnificent pieces with the natural host rock left on the back.

Boulder opals are fashioned to standard shapes and sizes but are mostly cut in freeform shapes to highlight their individual beauty and to avoid unnecessary wastage.

A full range of colours swirls and flashes in the depths of a light opal. The background colour may be white or light blue. Light crystal opal is translucent and shows colours sharp and visible below the surface. When clear and colourless, this form is referred to as 'water' or 'jelly' opal.

More recently, Mintabie has discovered the more vibrant semi-black opal, which is tremendously popular for its beauty, and not as expensive as black opal. Light opal has a lighter background while semi-black opal has grey colour tones.

Doublet Opal
A doublet opal is not a solid opal: doublets are made of thin slices of fine quality opal (generally light opal) glued to a backing piece of black potch, glass or Queensland ironstone, thus resembling natural black opal or Queensland boulder opal.

Triplet Opal
A triplet opal is not a solid opal: triplets are made of three pieces, rather like a sandwich. Firstly, a flat thin slice of precious opal is glued on to a backing of common opal, glass or porcelain that has first been darkened. A protective dome of clear quartz crystal is then cemented to the precious opal with a clear resin (glue) to complete the triplet opal. A doublet opal is more valuable than a triplet because it has a greater opal content.

Common Opal
'Common opal' is classified as non-gem quality opal. There are several varieties of common opal; most are opaque and none exhibit any 'play of colour'.

'Hyalite', or 'Mullers Glass', is a colourless opal that gives the appearance of glass. Rarely, it does display a faint tint of colour (blue, green or yellow)

'Hydrophane' is an opaque porous opal that becomes transparent when immersed in water.

'Resin opal' is black or brown with a resinous lustre.

'Potch' is generally opaque and can be milky white, pale to dark grey, bluish grey or black. 'Magpie potch' is made up of black and white patches. A clear amber variety of potch has been found at Lightning Ridge.

Opalised Fossils
The Australian opal fields were at one time under the sea, so opalised fossils are occasionally unearthed. As the ages passed and the seas began to recede, millions of sea creatures were isolated and marooned. Eventually the area dried completely and the inland is now a dry desert country.

In time the ground waters, holding silica solution, also evaporated. They left behind the phenomenon known as 'opal'. Hydrated silica was deposited in fissures in sandstone, or gypsum, and on jasper. It entered the shells of the stranded marine creatures. In some cases it even replaced the entire shell.

One can find opalised wood, prehistoric animal bones, sea creatures, full sea shells, skin shells, sponges, fish skeletons and even opalised stems of plants on the opal fields.

Synthetic Opal
Synthetic opal is produced in the laboratory and has a similar structure to that of precious opal. The following observations can be made to differentiate between natural and synthetic opal:

• Synthetic stones show brighter colours, and larger colour patches than in natural opal.
• In synthetic opal, colour grain boundaries are highly irregular.
• Synthetic opal has a distinctive snakeskin pattern.
• Synthetic material shows a more ordered array of colours because the intricate pattern of natural opal cannot be duplicated.

Imitation Opal
This is non-opaline material such as coloured tinsel set in clear plastic or epoxy resin.