Why are opalised shells so important?

Because opal has a microcrystaline structure, which will preserve very fine detail, opalised shells provide a great deal of information to the paleontologist.

About 110 million years ago there was a shallow sea which covered much of inland Australia. It is in this sea that many extinct creatures were spawned. We know they existed here from the fossil record of the area.

The varieties of fossils found in these ancient beds, include molluscs such as cockles, mussels, clams and sea snails, belemnites, which were squid like animals that looked similar to cuttlefish, crinoids or sea lilies and some now extinct species of marine reptiles - Ichthyosaurs, Pliosaurs and Plesiosaurs.

Probably the most fascinating, and widely known, of all the opalised fossils are the opalised shells found on the opal fields of Australia, particularly at Coober Pedy.

Opalised shell fossils are unique to the Australian opal fields and top quality shells command high prices.

During this period of the early Cretaceous era cockles were the most common of the molluscs present but other varieties of shell, such as mussels, were also abundant in the Eromanga Sea, as it is known.

How Are Opalised Shells Formed?

The Eromanga Sea was also home to many now extinct marine creatures and may have been the breeding ground for marine reptiles. This is thought because of the number of juvenile fossils found.

When marine animals die, their remains sink to the bottom of the seabed.

The fleshy parts may be eaten by other creatures but the bony or hard calcium remains may become buried under sand or silt and are preserved. With the build up of sediments over millions of years those remaining bony or shell parts may become fossilised.

In some cases the shells, being calcium carbonate, may be dissolved by acids in the soil, leaving cavities which may, in turn, be filled with other minerals, even opal. Sometimes, the cavities are filled with precious opal and these are highly prized amongst collectors.

About 65 Million years ago, the various species of sea reptiles perished along with the dinosaurs, but many of the molluscs survived and their ancestors are still surviving to this day.

It is not known why some species died out completely but the survival of those that remain today could, possibly, be likened to the process whereby an embryo is frozen and later thawed to propagate the species which later evolved into the varieties we see along our shorelines to this day..

The opalised fossils are found by miners, known as gougers, who work the opal fields for precious opal. Opal can be more valuable than diamond and it is the promise of a 'Lucky Strike' that tempts miners away from city life to the harsh and unforgiving conditions encountered in the hot arid regions of Australia's outback.

Shells which contain precious opal fetch very high prices. Dealers value the quality of the opal and then the price is adjusted to take into account the fact that it is an opalised fossil.

Most of the opalised shells are white opal and can be valued up to $1,000 each but crystal opal shells are far more valuable and can be several hundred dollars per carat.

An opalised shell which doesn't have any play of colour is known a 'potch' shell.

Minerals and Fossils

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