Opal is hydrated silicon oxide. Unlike quartz (silica) which is pure silicon dioxide, opal has some water in its structure, and takes the form of microscopic spherules of various sizes. When these spherules accumulate in a loose jumble the opal is nondescript and may occur in a wide variety of colors. This is known as 'common opal'. However, in rare cases the spherules are all approximately the same size and form evenly-spaced, uniform layers. These layers are thin enough to diffract light, creating a rainbow of colors known as 'fire'. Opal displaying fire is termed 'precious opal'. This is found in many localities, but the best known of all is Australia, and opal is that country's national gemstone. Opal occurs typically filling voids in volcanic rocks, and also as nodules and seams in sedimentary rocks such as sandstone. The precious opal from New South Wales, Australia is world famous and an important gemstone material.