Opal is the modern October birthstone and the accepted gem for the 13th wedding anniversary. Most opals are between 50-65 million years old, dating back to the Cretaceous period when dinosaurs roamed the earth. Opal formed as silica from decomposing rocks mixed with ground water which formed a silica gel that collected and hardened in underground cavities and fissures. There are two distinct types of opal, common and precious. The way the silica particles form …showed first 75 words of 414 total…

…showed last 75 words of 414 total…The uses of the opal can be restricted because of crazing. Crazing is the breakdown or deterioration of opal by the development of very fine cracks all over the surface that extend until they intersect. In the worst cases, the surface of the opal deteriorates into a crumbling sand-like material. Arab cultures believed that Opals fell from heaven in flashes of lightening, and other ancient cultures thought Opals were good fortune and banished evil spirits.