Fossils – rare collector’s pieces

Finding an opal is always a thrilling moment, but sometimes a particularly spectacular find will leave even the most experienced opal miners positively thunderstruck. Should a lucky miner discover a “thunderbolt,” he has especially good reason to react in this way: fossilised rostra, or the skeletal elements of fossilised cephalopods, are traditionally called “thunderbolts” or “thunderstones.” These extremely rare bones, fossilised in precious opal, have been left behind by belemnites, creatures similar in appearance to today's squid, which lived millions of years ago in an ocean which once covered part of present-day Australia.

During petrification, organic material becomes fossilised through a process of replacement and permineralization. When opals developed, silicic acid, the basic material of all opals, sometimes accumulated in cavities in the stone, created by the decomposition of living beings. With lots of luck, all these processes perfectly combined to create opalised fossils. This process is the reason why cephalopod fossils, which were formed in the Cretaceous period many millions of years ago, can sometimes be found in opal mines. In addition, other opal fossils, such as mussel shells, snail shells, bones and vertebrae, and rarely even near-complete preserved skeletons of dinosaurs or very early mammals. Even opalised wood has formed over the years, and can still be found.