Although comprised simply of silica and water, opal is one of the most complex gemstones known. Not only are opals unique among the members of the mineral family, no two opals are identical. The molecules in most gemstones form crystals, but the silica molecules in opals take the shape of tiny spheres. The silica spheres pack together tightly in precise layers. Although the spheres and layers are as close to each other as they can get, tiny spaces between them exist. Opal's uniqueness results from the minute differences in the sizes of the silica spheres and their spacing from one another.
The play of color, or fire, prized so highly in precious opal, is a special effect of light. All color comes from light, which may be absorbed or reflected back from solid substances like gemstones. The absorbed colors, "bouncing" around the silica spheres and the spaces between them, produce a brilliant play of color as light moves across an opal's surface.
Each type of color is named according to its appearance. Large square-shaped patches of color are called harlequin or mosaic. Flash fire describes large irregular patches of color. Flame fire is streaks of color, while pin fire refers to small pinpoints of color. Sheets of color that roll across the surface as the stone is moved are called rolling fire. Broad flash fire is sheets of color that cover the stone. Other descriptive names include zebra, Chinese writing, and straw. These latter varieties are rarely found.
Opal that doesn't have a play of color is called common opal or potch. Water, or jelly, opal is transparent, without a play of color, but has an iridescent glow that makes it desirable.
Gilson opal is a synthetic (manmade) which mimics all the chemical and physical properties of genuine opal. Slocum stones are imitation opals made from glass. Both glass and plastic imitations have been named Opal Essence. Opalite is made from plastic. Many of the imitations duplicate the play of colors so skillfully that they are difficult to distinguish from precious opal with the naked eye.
Although nearly all precious opal today comes from Australian mines, opal is found throughout the world.
