Gem quality opal is one of the most spectacular gemstones. A single stone can flash every colour of the spectrum with an intensity and quality of colour. Opal has a hardness of about 5.5 to 6.0 on the Mohs hardness scale, softer than most other gemstones. Because of that opal works best in earrings, brooches and other pieces that rarely encounter skuffs and impacts. Opal is the national gemstone of Australia, accounting for 95-97% of the world’s supply of precious opal. Recent data suggests that Ethiopian opal production has been increasing of late.
Most of this opal is “common opal” or “potch” which has a milky or pearly luster known as “opalescence”. However, rare specimens of opal produce brilliant colour flashes when turned in the light. These colour flashes are known as a “play-of-colour”. Opal specimens that exhibit a play-of-colour are known as “precious opal.” A play-of-colour in opal can be observed under three situations: 1) when the stone is moved, 2) when the light source is moved, or, 3) when the angle of observation is changed.
The “Aurora Australis” was found in 1938 at Lightening Ridge and is considered the world’s most valuable black opal. The oval, cut and polished stone has a harlequin pattern with dominant red, green and blue colours against a black background. It weighs 180 carats and is 3 inches by 1.8 inches. The rarity of the opal comes from its size and strong, vibrant colour play. Dug from an old sea-bed it has the distinctive impression of a star fish on its back. It was valued at AUD$1,000,000 in 2005.
Credits: Geology, Opals Down Under, Wikipedia.