What is the significance of opal?

Opal is Australia's national gemstone, proclaimed by Governor-General Bill Hayden on 28 July 1993. The country produces about 90 per cent of the world's commercial opal supply, mined principally at Coober Pedy, Andamooka, and Mintabie in South Australia, Lightning Ridge in New South Wales, and Quilpie and Yowah in Queensland.

Opal forms when silica-rich water seeps into cavities in sedimentary rock and slowly deposits layers of microscopic silica spheres over millions of years. The way these spheres diffract light produces opal's distinctive play of colour, with different deposits showing different palettes. White opal (predominantly from South Australia) shows pale colours on a milky background. Black opal (predominantly from Lightning Ridge in New South Wales) shows brilliant colour against a dark background and is the most valuable form. Boulder opal (mostly from Queensland) shows colour in veins running through ironstone host rock.

Opal mining is concentrated in remote outback towns that have developed a distinct culture of small-scale operators. Coober Pedy, founded in 1915 about 850 kilometres north of Adelaide, is famous for its underground homes (called dugouts) carved into sandstone to escape summer temperatures that regularly exceed 45 degrees Celsius. Lightning Ridge, in north-western New South Wales, has a population of about 2,000 and produces almost all of the world's black opal. Mining is regulated under state legislation and is open to individual prospectors as well as commercial operations.

Aboriginal peoples have collected opal for ornamental and ceremonial use for tens of thousands of years. The Andamooka Aboriginal Lands and the Lightning Ridge area are both associated with traditional Indigenous use of the stone. The Andamooka Opal, a 203-carat stone presented to Queen Elizabeth II during her 1954 royal tour, and the Olympic Australis, a 17,000-carat specimen found at Coober Pedy in 1956, are among the most famous opals in the world. Opal is now the official gemstone of South Australia (1985), New South Wales (the black opal, 2008), and the Australian nation.