Centenary of the Australian opal: the rainbow effect

A hundred years ago, on May 8, 1915, the first piece of opal was found in Australia. Willie Hutchinson was prospecting for gold with his father in a desert town called Coober Pedy when he found a few pieces of whitish iridescent rainbow-coloured stones. Coober Pedy is now the “opal capital of the world.” It produces 80% of the world’s opals (BBC, May 8, 2015).

A group of international geologists hope to legally define the opal, Australia’s national gemstone, as a Global Heritage Stone Resource (GHSR) in order to recognize and preserve the opal’s significance in human culture. No stone has yet been designated a GHSR status.

Initially the geologists wanted to recognize specific construction rocks, such as Portland stone, Carrara marble, Sydney sandstone, and Norwegian larvikite. They want to designate and protect particular stones and rocks for conservation. But when the opal was added to the list, it was a controversial decision, even amongst geologists. Some stated that opal was not a construction stone because it was processed as jewellery.

The centenary of opals aims to highlight the magnificence of their geological context. They result from the Eromanga Sea that covered central Australia about 100 million years ago – it was the great central sea. Opals in Coober Pedy are derived from sedimentary effects. Acidic fluids dissolved silica from sandstone rich in quartz, which gives the stone iridescence – a rainbow effect. Opals found in other locations originate from volcanic events, not sedimentary settings.

The Global Heritage Stone Resource project commenced in 2008. The project has not yet designated its first status to any stone – but the honour is likely to go to Portland stone, the cream coloured building limestone found in quarries in Dorset on the south coast of England. St. Paul’s Cathedral and Buckingham Palace in London are constructed from Portland stone, and so is the United Nations building in New York. So the opal, a controversial choice, may take another hundred years before it makes the GHSR status as a heritage stone.

MARTINA NICOLLS is an international aid and development consultant, and the author of:- The Shortness of Life: A Mongolian Lament (2015), Liberia’s Deadest Ends (2012), Bardot’s Comet (2011), Kashmir on a Knife-Edge (2010) and The Sudan Curse (2009).