Examples of opalescent fossils include remnants of ancient plants, mussels, snails, crustaceans, fish, dinosaurs, birds, mammals, and can date from 100 million to 250 million years old.
The most sought after of these fossils come from the Lightning Ridge area of Australia, which was once a shallow inland sea. In most cases, these fossils are made up of silica which has accumulated in the shape left by these creatures, as with a jelly mould. They tend to be small fragments of this organic material and complete examples are rare.
Ammonites are the most common of this type of fossil and were cephalopods, or squid-like creatures, that thrived in tropical seas, until becoming extinct, along with the dinosaurs, at the end of the Mesozoic era. These are mined commercially in North America and Canada and were given official gemstone status by the World Jewellery Confederation in 1981, under the title of “Ammolite”.
Strictly speaking, the iridescence on these fossils is more to do with the lack of acid in the soil than the formation of silica to form true opal. The colours, mainly green, red or blue, are caused by light passing through the various layers of aragonite and conchiolin [shell layers], grown by the Ammonite in life.
The examples I am offering for sale are from Madagascar.