The opal is a fragile hydrated silica material, made of submicroscopic silica spheres held together by more silica and water. It is a soft stone, easily altered in appearance by changes in heat and pressure. This mineral contains varying amounts of water within it that determine the appearance of the gemstone. When water evaporates out of an opal, the stone appears slightly smaller and the stress of the evaporation creates cracks on it. Opals are formed in near-surface volcanic rocks, within cavities and cracks.
Opals also have characteristic colors due to impurities within the stone.
The milky or pearly appearance of some opals are due to inclusions of tiny gas bubbles. Yellows and reds betray the presence of iron oxides. The spectacular black opals that sometimes flash green, blue and red get their color from magnesium oxides and organic carbon within the stone. The principal source of opals is Australia, noted for its magnificent black opals. Fire opals were first mined in Mexico, and continue to be produced today.