The Opal is an amorphous form of silica related to Quartz. It's not actually considered a mineral but a mineraloid, which is a mineral-like substance. It's internal structure makes it diffract light. Depending on the conditions in which it is formed determines what colors it will end up. Opal is the national gemstone of Australia, which also produces 97% of the worlds supply. When it comes to jewelry, beware of synthetic opal. When held under magnification, synthetic opal will have patches of color in a pattern such as lizard skin or chicken wire so be sure to inspect the gem carefully before purchasing.