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Opal is one of the world's most rare and valuable gemstones, and it comes in every colour of the spectrum. Opal is made of tiny, microscopic spheres of silica. It is these tiny spheres that give opal its amazing colours, by breaking white light up into rays of different colours. When white light enters a precious opal, it hits the spheres of silica. The spheres split light into rays of different wavelengths, which we see as different colours. The colours and patterns in an opal are determined primarily by the size and arrangement of the silica spheres it contains.
Opal forms within spaces in rock. It is found within sedimentary rocks, such as sandstone; and in volcanic rocks that have hardened from molten lava with air bubbles trapped inside. The basic ingredients for opal are water and silica - but the exact conditions required for opal formation are still not fully understood. On the extremely rare occasion that conditions are just right, silica collects in spaces within the rock and gradually hardens to form opal. The opal takes the shape of the space it is filling. Where the rock cavities once contained plant or animal remains, opalised fossils are formed. More than 95 per cent of the world's opal is found in Australia. Most of this opal is found in the Australian outback, around the margins of an ancient inland sea that once covered almost one third of Australia - around 110 million years ago, during the Cretaceous period. The opal is found in small pockets between layers of sandstone in clay beds that once formed the floor of this great lost sea. Some scientists think this Australian opal took thousands of years to form, at high temperatures and under great pressure; others think the opal formed relatively quickly, at about 20 degrees celcius. It seems that at least some opal formed when bacteria took silica from the surrounding clay or stone, then deposited it in spaces in the rock, where the silica became opal.
Scientists continue to try to understand the details of how opal forms - and opal miners follow this research with great interest, in the hope that knowing how opal formed will give them a better chance of knowing where to find it. Opal mining is just about the only mining enterprise still run by individual operators or small partnerships, rather than by large companies. This is mainly because opal is so rare, and occurs so unpredictably. Despite its rarity (indeed - because of it!), opal is extremely valuable, and is an important export product for Australia. It is also this chance of 'striking it rich' that inspires opal miners to stick to their tough and risky occupation. The equipment used for opal mining depends on whether the opal is mined underground or by the open cut method; and also on the miner's budget. Equipment ranges from hand picks through jackhammers to small machines called diggers and boggers; right up to large earth moving equipment. Most miners start out with the basics, then upgrade when they strike opal. Most opal is mined from underground, where it formed. In Australia, opal is found down to around 25 metres below ground level. Occasionally opal is found on the surface after it has been brought there by years of weathering and erosion. In some areas of Australia opal is mined by the open cut method, whereby all of the ground above the opal is removed to expose the 'opal level ' - the layer of rock in which opal may be found. Material from this layer is known as opal dirt. Where opal is deep underground or is found in only small, elusive patches, the open cut method is uneconomical. Moving such large amounts of earth is too expensive unless there is a good chance of finding a relatively large amount of opal.
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Australian opal is classified into five main types: black opal, light opal, crystal opal, boulder opal and common opal.
Light opal contains gem colours sparkling from a white or pale-coloured body. Light opal is mined mainly at Coober Pedy, Mintabie and Andamooka in South Australia and at White Cliffs in New South Wales; some light opal is found at Lightning Ridge.
Opal direct from the mining or washing process is called rough opal. An opal cutter will take a piece of rough and first use an abrasive lapidary wheel to carefully rub away the opal's covering of stone or potch, to reveal exactly where the gem colour is. At this stage, the opal is called a rub. The opal cutter shapes and polishes the opal gemstone using a series of increasingly-fine abrasives and polishes. With each pass over the cutting wheel, the cutter reassesses factors such as the location, depth and direction of the band of gem colour; the presence or removal of impurities; the brilliance of colour, the patterns it forms, and the way these change within the stone. Removing a fraction of a millimetre may reveal the brightest face the stone can have.....but it could also ruin the stone forever. The opal cutter's job is a highly skilled one, involving a constant process of feedback between eye and hand, informed by experience and by sensitivity to this marvellous and unique gemstone. The rewards are the thrill of discovery - each opal gemstone is different to the last, its secrets waiting to be revealed - and the satisfaction of crafting rare and exquisite opals, each one absolutely unique. Every opal contains an infinite number of hidden faces. It is the cutter who determines which face an opal shows to the world.
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Black opal is opal with gem colour on a natural dark base. The dark base colour shows up the gem colours more brilliantly than in any other kind of opal. When the base colour is dark grey rather than black, the opal may be referred to as 'semi-black' or 'grey'. Black opal is the most rare and valuable opal type. Nearly all of the world's supply of black opal is mined at Lightning Ridge in New South Wales.
Crystal opal contains gem colours in a transparent or semi-transparent body. Crystal opal can be extremely beautiful, and is often used to make opal doublets or inlay jewellery. It is mined mainly at Andamooka in South Australia and at Lightning Ridge in New South Wales.
Boulder opal is gem-coloured opal found within a body of ironstone or iron-laden sandstone. It is named for the boulder-shaped lumps of ironstone in which it is often found. The dark-coloured stone gives some boulder opal a brilliance similar to that seen in black opal - but the gem colour usually occurs in thin, wavy sheets, so boulder opal is often cut with rock matrix showing in the face.
Common opal is opal that does not contain gem colours. Common opal has the same chemical composition as gem opal; but whereas gem opal contains microscopic spheres arranged in a regular array, in common opal the arrangement of spheres is irregular. The best-known kind of common opal is potch, which may be black, grey, white or amber-coloured. A natural backing or body of black potch is a defining characteristic of black opal (see 'Black opal' above).
