Our world produces some incredible rocks. Take the opal, a precious stone that forms out of silica, in the dark under the surface of the earth. When cut and polished, it flashes with a gorgeous array of colours, from pale milky hues to deep reds and blacks.
Now, an opal that has been named the finest ever unearthed will be making its public debut next month as the centrepiece of an exhibition opening at the South Australian Museum, Adelaide.
Called the Virgin Rainbow, it was discovered in the opal fields of Coober Pedy by opal miner John Dunstan in 2003.
It's actually an opalised fossil, from an extinct cephalopod called belemnitida that existed during the Mesozoic era. During that time, much of South Australia was under a vast inland sea filled with prehistoric aquatic reptiles called plesiosaurs. These died and sank to the bottom of the sea, buried over the millennia by sediment.
When the sea dried up and the land turned into a desert, the acidity levels in the shallow top layer of the sandstone increased. This released silica from weathering sandstone into the layer of clay beneath, where bones and pockets left by disintegrated bones lay buried, carried down via groundwater.
Further weathering lowered the acidity levels, which allowed the silica gel to harden into opals in the pockets and impressions left by decayed animal material, like cake poured into a mould, or to soak into bones and create a replica of the internal structure.
The famous Australian opal fields of Coober Pedy are located in this region. No other environment in the world is known to have undergone this same process, which is possibly why over 90 percent of the world's opals come from South Australia.