Opal is the most magnificent of precious gemstones.
It boasts every colour of the visible spectrum, from deepest and clearest blues and greens to rippling golden orange; through delicate pink and violet to rich turquoise, shocking vermilion, carmine and fuschia – every colour imaginable.
An opal may contain any or all of these colours, arrayed in wondrous patterns with names like harlequin, pinfire, Chinese writing, flower garden, mackerel sky, flagstone and rolling flash. Like all fine things, gem opal is exceedingly rare. Much of the world’s precious opal is mined in the harsh outback of Australia, where a unique combination of geological conditions permitted the formation of opal near the margins of an ancient inland sea. In 1994 opal was declared Australia’s National Gemstone.
Elsewhere in the world, opal is found in Brazil, Mexico, Ethiopia, the United States, Canada, Peru, Indonesia, Honduras, Slovakia and the Czech Republic. Each location produces a distinctive opal type; in the future, we will add photos and information about these to the Australian Opal Centre web page.
To learn more about the physical and gemmological properties of opal, a good place to start is the nomenclature page of the Opal Association website. To whet your appetite, here is some solid Australian opal.
Black opal has what is called a black body tone. If you ignore the colour in an opal, how dark is it? If it is black or near-black, it is called a black opal.
Black opals come in every colour of the rainbow. Their dark body tone makes the colours on the face of the opal appear rich and intense. Opal with a dark, but not black, body tone, is referred to as dark opal.
Black opal is the most rare and highly valued form of opal.
In light opal, there is a play of colour within, or on, opal that has a light body tone.
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 hold it to the light or place a light source behind it, some light will pass through.
When you look at crystal opal with a light shining through from behind, the body of the opal takes on a warm orange colour.
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-coloured, iron-rich rock called ironstone. If the opal is in a thin layer on dark ironstone, it looks rich in colour and dark in body tone, like black opal. Sometimes the opal is distributed in thin, irregular veins throughout the ironstone. This is called boulder matrix.
Boulder opal country also produces thicker ‘pipes’ of crystal opal, formed when the opal infilled cylindrical cavities left by burrowing invertebrate animals, tree roots or fallen branches.