Opals form when silica gel hardens in the cracks of other rocks. Most of the world's precious opal comes from Australia, where opal is the national gem. A boulder opal is an opal that has been sliced together with its surrounding ironstone for a varied look. Fire opal from Mexico comes in shades of orange and Peruvian opal is an opaque blue-green stone. An opal doublet has polished precious opal on the top with an ironstone backing added for strength, making the slice of opal less prone to chipping.