Opalized Fossil

There is a very rare and special type of gemstone that can only be found and formed in fossils of extinct species of cephalopods over the process of hundreds of millions of years. The most expensive opal in the world is one of these, being an opalized fossil valued at over $1,000,000. There is also a six-meter Plesiosaurus that has an opalescent sheen in its skeleton on display at the Adelaide Museum.