Opals are a form of hydrated silicon dioxide, formed by a combination of water and silica. In fact, opal stones consist of 3-10% water. Their unique properties are due to the way the microscopic silica spheres are composed in the stone. The composition creates the remarkable play of colors that characterizes opals. Opals are found in different parts of the world, but Australia is the largest source, supplying over 90% of the world's finest opals. The stone is considered to be Australia's national stone.