How were opals created? 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 deposit. The deposit eventually hardens to form common opal, and in rare circumstances it forms precious opal.