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 will just make it complicated in explaining. Opal is a non-crystalline form of the mineral silica and is formed from amorphous "balls" or "lumps" of silica.
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 .
Natural Opals are made up of: Natural â€“ Light, Dark and Black; Natural â€“ Boulder; Natural â€“ Matrix. Man-Made Opals include: Man-Made – Treated Matrix .