The greatest occurrences of opal fossils are mainly found in and around Coober Pedy, with smaller finds at other locations in South Australia. Sea shells, shell skins, belemnites(pipes) and sea snails are the main types of opalised fossils found.
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The formation of opal fossils began when marine life was buried below sand and silt at the bottom of the inland sea which covered much of southern and eastern Australia during the Paleozoic and Mesozoic eras, over 200 million years ago. When the marine life decayed after many years below the earth's surface it left cavities that were later filled with the silica rich waters from the inland sea that produced today's fossil opal