Opal is Australia’s national gemstone, and the most magnificent and colourful of precious gems. Like all fine things, precious opal is exceedingly rare.
Opal can boast any colour of the visible spectrum, from deep clear blues and greens to golden orange; from delicate pink and violet to rich turquoise, shocking red or fuschia.
An opal may contain any or all of these colours in wondrous patterns with evocative names like harlequin, pinfire, Chinese writing, flower garden, mackerel sky, flagstone, rolling flash, and others.
Even more amazing, the colours and patterns may change as the opal moves, creating a dance of light unlike any other seen in nature. This incredible phenomenon, called ‘play of colour’, is unique to opal.
The majority of the world’s precious opal is mined in Australia’s harsh outback, in the states of New South Wales, Queensland and South Australia. Many millions of years ago, a unique combination of geological conditions around an ancient inland sea permitted the formation of the world’s greatest opal deposits.
What causes opal's play of colour?
Unlike the colours of most other gemstones, colour in opal isn’t caused by mineral inclusions. (For example, the blue of sapphire is usually caused by its iron content, and the red of ruby by the chromium it contains.)
Instead, the colours we see in opal are a magician’s trick using balls and light. The magician is nature, giving a mind- (and light-) bending performance on a microscopic scale.
The main ingredient in opal is silica – the most abundant mineral in the Earth’s crust, and most commonly found as sand or quartz. Silica is also the main ingredient in materials such as glass, concrete and even toothpaste.
Opal consists of tiny spheres of silica dioxide, measuring between 150 and 300 nanometres in diameter. (A nanometre is one millionth of a millimetre.) If the spheres are arranged in neat arrays, light hitting the opal is split into its component colours before it bounces back to our eyes.
This phenomenon of interference and diffraction causes opal’s amazing play of colour. It’s like light passing through a prism, or raindrops making a rainbow from sunlight.
Opal with an orderly array of silica spheres and play of colour is called ‘precious opal’. If the spheres are jumbled, there is no play of colour and the opal looks grey, black, white or amber-coloured. Called 'common opal' or ‘potch’, this accounts for over 90% of opal found.
The size of the silica spheres determines the colours in the opal: the colours you see in a precious opal are produced by white light being diffracted by spheres of different sizes, in neat arrays, in precious opal.
The four main types of Australian opal
Black opal has a black body tone and is the most rare and highly valued form of opal. Found in every colour of the rainbow, black opal's dark body tone gives depth and richness to the colours it displays. Opal with a dark, but not black, body tone is called dark opal.
The gem colours in light opal have a beautiful soft, pastel quality. The lightest of light opal is also sometimes called white or milky opal.
Crystal opal is translucent: if you place a light source behind it, some light will pass through. Although most crystal opal has a light body tone, sometimes it can be as dark as black opal. In that case, it is called black crystal.
Boulder opal forms in cavities within a brown, iron-rich rock called ironstone. A thin layer of opal on dark ironstone can look rich in colour and dark in body tone, like black opal. If the opal occurs in fine veins throughout the ironstone, the stone is called boulder matrix.
Opalised fossils: Precious relics from the Age of Dinosaurs
Australian opal mines sometimes produce rare 110-million-year-old fossils from the Early Cretaceous period. At that time, dinosaurs and other ancient creatures lived in mighty forests that grew where Lightning Ridge is today; and marine reptiles swam in a shallow sea over much of inland Australia, home now to the opal mining towns of White Cliffs, Coober Pedy, Andamooka and Mintabie.
These are no ordinary fossils, but incredible relics made of solid opal, sometimes with rainbows of shimmering colour. Australia is the only place on earth where opalised animal fossils are found. Australian opalised fossils are of global scientific interest and are among the most rare and beautiful fossils in the world. They are national treasures.
Opal mining: The search for one of Earth’s great treasures
Opal is mined from sedimentary deposits in outback New South Wales, South Australia and Queensland. Anyone can become an opal miner and, with hard work, skill and good fortune, have the chance of finding opal.
Opal mining is conducted mostly by underground shaft and tunnel methods, with the mine accessed via a vertical shaft about a metre in diameter and up to 25 metres deep. From the bottom of the shaft, the miner digs horizontally to form tunnels called drives, searching for opal. Open cut methods are also used in some areas.
The technology of opal mining is fascinating. Every Australian opal field has produced innovations in response to its particular geological, technological and social settings.
For decades, opal mines were dug by hand using picks and shovels. Dirt was hauled to the surface in buckets, tied to ropes; a windlass could make the job easier.
Today, Calweld drills are used for sinking shafts. Miners dig with jackhammers, hydraulic diggers or tunneling machines; 'opal dirt’ can be raised to the surface using an automatic hoist or a ‘blower’ (which sucks the dirt up through a pipe); and an ‘agitator’ may be used to isolate hard tailings (hopefully including opal). Open cut mines are worked using earthmoving machinery.
Opal mining is conducted on a small scale, with opal claims owned by Individual miners or small companies. Miners work alone or in small partnerships, following strict safety and environmental rehabilitation standards. In New South Wales, each claim is no larger than 50 x 50 metres and a miner may have no more than two claims registered at any one time.