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.
We already know that opal is made from silica combined with water. Some other components have been found inside opal, but we think these other components .
Areas within an opal that produce a play-of-color are made up of microscopic spheres of silica arranged in an orderly network. This network of spheres acts as a .
An opal is a 'gemstone' – that is, a mineral valued for its beauty.. . Large slices of opal rock are made into very expensive, wall panels, tiles and coffee tables.