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 thousands of 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. Shells of stranded marine creatures, prehistoric animal bones and even stems of plants have been left opalised. Many opals have been found in Lightning Ridge becoming known as Black Opal fossils.