Opals are formed from a solution of silicon dioxide and water, as water runs down through the earth it picks up silica from the sandstone, and cares this silica rich solution into cracks and voids cause by natural faults or decomposing fossils. as the water evaporates, it leaves behind a silca deposit. This cycle repeats over very long periods of time eventually an opal is formed. Opals formed during the Cretaceous period. Between 65-140 million years ago in a inland sea called the Great Basin.