National Gemstone of Australia
What is an Opal ?
Opal, is derived from the Greek “Opallos”, meaning ‘to see a change (of colour)’, it is a formation of non-crystalline silica gel.
Millions of years ago, this gel seeped into crevices and cracks in the sedimentary strata. Through eons of time and through nature’s heating and moulding processes, the gel hardened and can today be found in the form of brilliant opals.
The Opal is set apart from other gemstone, because of its characteristic appearance, displaying sparkling prismatic colours that change and flash as you turn the stone in your hand. No two opals are exactly alike.
Opal is found in many varieties and sub-varieties, but only a very small percentage of all opal found constitutes precious opal. This rarity is much greater than is generally appreciated, as the annual production of precious opal is infinitesimal compared to that of any other precious gemstone. Most opal found has little or no colour play – this type is known as “ potch”
Black Opal, is the rarest and most valuable type. It is mainly found at Lightning Ridge in NSW and some areas in South Australia. This magnificent variety of gem quality is arguably the most valuable. Ablaze with colour, its dark appearance distinguishes it from Light Opal.
Boulder Opal is a special type of opal found mostly in central Queensland. It is a curious formation of silica that has filled the cracks and crevices in light and dark brown ironstone boulders.
Invariably stones from Queensland are cut with the natural host rock (ironstone) left on the back. Sometimes, owing to the thinness of the opal seams, the boulders are sawn into baroque shapes, and polished with the brown host rock retained around the precious heart of opal. Small boulders sometimes containing “kernels” of opal are known as “Yowah Nuts” after the nearby town of Yowah.
Boulder “matrix” opal is a peculiar opal formation where flecks of rich, flash colours of opal are scattered throughout the brown ironstone, like twinkling neon lights of a distant city.
Light Opan is not quite as rare as the black varieties, but is nevertheless distinguished by lively colour paly. As the name suggests, the stone ranges from transparent, translucent to opaque with a creamy hue, displaying soft pastel shades of colour. The “crystal” translucent type is vibrant with shafts of colour in intricate pattern variations.
Found mainly in South Australia at Coober Pedy, Mintabie and Andamooka.
DOUBLET AND TRIPLET OPAL
A thin slice of colourful Light Opal is laminated onto a layer of dark material (usually Opal potch) to form a Doublet. The addition of a clear dome of crystal (where the Light Opal is sandwiched between the clear dome and the layer of dark material) makes it a Triplet. The capping gives the opal layer protection. Both varieties are water sensitive and hence stones should not be immersed.