Opals are delicate gemstones found in Mexico, Brazil, the United States, Japan and Honduras. The greatest proportion of the world’s opals, and those of finest quality, come from Australia. Up until the early 19th century, opals were considered rare, but use in jewelry flourished during the Art Deco period. Opals are still popular today and, in the United States, they are the birthstone for October. There are two forms of opal: common opal, which is mostly opaque, and precious opal, which exhibits play-of-color within the stone.
Play-of-color is the name for the display of shifting colors in an opal. It is caused by the clusters of microscopic silica spheres within the stone. When light hits these molecules, it breaks into different patterns and patches of color and iridescence. The patterns are given names to reflect their appearance. Harlequin has mosaic patches of roughly equal size and is the most desirable pattern. Flame opals exhibit patches of color that seem to roll and flash with the movement of the stone. Pinfire opals have glittering patches of color composed of many tiny points of color. Peacock is mainly blue and green.
Light opal includes white, crystal and jelly opals. White opal is the most common type. White opals with brilliant play-of-color retail for a few hundred dollars per carat. Crystal opals have a high transparency and the body is nearly colorless, with a distinct play-of-color. At $2,500 per carat, these are the most valued light opals, according to the Gemstone Buying Guide. Jelly opals range from nearly transparent to translucent with little to no play-of-color.
Black opals have a play-of-color against a black or very dark background. One of the main sources of top-notch black opal is Lightning Ridge in New South Wales, Australia. Today, top-grade radiant black opal can sell for $15,000 per carat, and some stones with exceptional, fiery play-of-color can retail for over $20,000 per carat.
Boulder opals are found in boulders whose cracks and spaces have been infiltrated with opal. This opal can resemble light or dark opal in appearance and is typically cut into irregular shapes. Boulder opal is very durable compared to other forms of opal. Its durability is due to the ironstone backing on which the opal forms. If you cannot afford a colorful black opal, a smooth, watery boulder opal can be a good alternative.
Mexico is the main source of fire opal, which is transparent to translucent and red, orange, yellow or brownish in color. Unlike most other types of opals, which are usually cut as round or oval cabochons or any softly domed shape, fire opals are well-suited to facet cutting, allowing them to reflect more light. The most valued and desirable fire opal is reddish orange, transparent, and has a vibrant, glowing play-of-color within the stone.
References and ResourcesInternational Colored Gemstone Association: Opal
Gemological Institute of America; Opal; Jan. 9, 2009
"Gemstone Buying Guide"; Opal; Renee Newman; 2005