Opal << OH puhl >> is a glassy material, some of which is valued as a gem for its flashes of color. Opal is one of the birthstones for October.
Much opal is white, gray, brown, or black. Some opal shows a brilliant flash of color, called play of color, on top of its background color. Opal with play of color is called precious opal. Opal without color play has little or no gem value, unless it is clear or brightly colored. Opal without gem value is called common opal or potch.
Opal consists of water and a mineral called silica, the most common ingredient in sand. Precious opal forms underground when water flows through cracks and spaces in the rocks. The water slowly evaporates, depositing tiny spheres of silica. If the spheres are the right size and in a uniform structure, they diffract (bend) light. Light diffraction within an opal removes some of the colors in light and strengthens others. This bending of light produces the play of color seen in gem-quality opal.
The value of an opal gemstone is mostly determined by its weight, its background color, and the color and extent of its play of color. The most valuable opals have a black body color with a vivid play of color dominated by flashes of red and violet. Opal with a dark body color is called black opal. The best-known source of black opal is New South Wales, Australia. Australia is also the main source of precious white opal. White opal has a white body color and usually has smaller patches of play of color that are often green, blue, red, and yellow.
Precious opal is less durable than most gems. It is relatively soft—it can be scratched with a knife—and can chip easily. In some opals, the water within dries up and the stone shrinks, causing tiny cracks. This process, called crazing or checking, lowers the value of opal.
Because the beauty of most opal lies in its color flashes, opal is rarely cut with facets (flat, polished surfaces). Instead, opal gemstones are cut with a curved upper surface. This style of cutting is called cabochon cutting. Opal with vivid body color, such as some fire opal, may be cut with facets.
Large opals include a Hungarian opal that weighs 21 ounces (594 grams) in the Museum of Natural History of Vienna, in Austria. The Roebling opal, found in Nevada, is a beautiful American opal. It is pitch-black with brilliant flashes of color. It weighs 18.7 ounces (530 grams) and is on display at the National Museum of Natural History in Washington, D.C. The Andamooka Desert Flame opal weighed over 15 pounds (6,800 grams) when found in Australia in 1969.
