It is very rare that a deceased creature will become fossilized. In even rarer cases, the deceased creature can become an opalized fossil.
Opal forms when water runs over a deposit of silicon dioxide (a major component of sand), mixes into a solution, drains into a crevice, and evaporates. Occasionally, the mixture of water and silicon will flow into a fossil cavity. The dried silicon becomes an imprint of the original object. This specific series of events leads to the creation of an opalized fossil.
Opalized fossils can only be found in Australia because silica is a remnant of ancient Australian volcano debris. Elsewhere, fossils can be preserved in limestone, agate, and pyrite.
But clearly, opalized fossils are the most beautiful.
Since these fossils are so rare, most are kept at the Australian Opal Center or Australia's National Opal Collection.
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