Opal is formed 20- 100 million years ago, when there was a big inland sea in Australia. Without going in to too many details, opal is formed from a solution of silicon dioxide and water. As water runs down through the earth, it picks up silica from sandstone, and carries this silica-rich solution into cracks and voids, caused by natural faults or decomposing fossils. As the water evaporates, it leaves behind a silica deposits. This cycle repeats over very long periods of time, and eventually, opal is formed. Some of the voids has been plant material or animal bones. That is why we sometimes can find opalised bones, teeth and shells from prehistoric time.