Spencer Opal Mines

The Authority in Opal






About Spencer Idaho Opal

In Idaho, Opal is the second largest contributor to the total value of gem material produced. The varieties produced include precious (white and pink), yellow, blue, pink, and common. The Spencer Opal Mine, the largest privately owned gem stone producer in the State, is the major producer of Opal.


The most important and most widely known opal is the precious opal. Precious opal may be subdivided further by color modifiers, white, black, pinks, and blue, which describe the body color of the opal. Opal is the most colorful of all gems. Its splendid play of color is unsurpassed, and fine examples can be more valuable than diamond. The play of color consists of iridescent color flashes that change with the angle at which the stone is viewed. This phenomenon is often called opalescence. The play of color may consist of large, individual flashes of color (known as schillers), or may be of tiny, dense flashes. The intensity and distribution of the color flashes is a determining factor in the value of an opal. A condition called crazing affects certain opals, causing them to form internal cracks. Crazing is a particularly interesting phenomenon, for it lacks consistency and is unpredictable. Although it can occur at random, it usually strikes when an opal removed from damp conditions is allowed to dry too quickly, or when an opal is exposed to sudden intense light -- or a combination of these factors. Crazing may also take place when an opal is subject to vibration, as during the cutting and polishing of a specimen. The severity of the crazing and the time it takes to "craze" varies among specimens. The origin of the specimen is often a determining factor to its resistance to crazing. A very gradual drying process over months or even years can in some cases effectively stabilize the stone and allow it to be cut and polished with a substantially reduced risk of crazing. Opals displaying play of color are known as precious opals, and opals lacking play of color are known as common opals.

16.2Gems are cut from both the precious and common forms, but precious opal is the primary gem form of this stone. There are many varieties of both precious and common opals. The most desired and beautiful form of opal is black opal, which is opal with a dark blue, dark green, or black background with a strong play of color. Next in importance is white opal, which is opal with a light colored body color (white, yellow, cream, etc.) with strong play of color. Next in line is Mexican fire opal, a transparent to translucent orange red form of common opal. If Mexican fire opals display play of color, they are known as Precious Fire Opal. Many precious opals, besides being classified as either black or white opals, are further classified based on the distribution and habit of their play of color. There are also many variety names given to the numerous forms of both precious opal and common opal.

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