Opalised pearls Most of the best opals are the relics of…

Opalised pearls

Most of the best opals are the relics of complex geochemistry in the remnants of a long gone inland sea in Australia some 65 million years ago at the end of the Cretaceous, and most of the nice opalised fossils are found there (see http://bit.ly/2gO6joS for an explanation of how they formed, and http://bit.ly/1TFM2iH and http://bit.ly/2xReZBD for a couple of nice examples). Many shells and even plesiosaur bones have turned up in the mines of Coober Peedy over the years, but this pair of fossilised pearls measuring around half a centimetre each that dropped out of long gone shells onto a vanished seafloor and turned into precious silica many millions of years later is a first…

They were found in a spoil heap using UV light, and their unusual shape attracted the miner’s attention. Confirmation of their nature involved sophisticated non destructive scans, using the neutron imaging instrument (similar to a CT scanner) at the Australian Nuclear Science and Technology Organisation at Lucas Heights in Sydney. It revealed the concentric layers typical of natural pearls (the second image), which grow by precipitation of layers of nacreous aragonite crystals around an irritant. These beauties are on temporary display at the South Australian museum, and miner’s eyes will be peeled for more such beauties. Even in inferior qualities, opal fossils carry a cachet (and premium) in the gem trade.


Image credit: South Australian Museum