Opal is the most colorful of gems. Its splendid play of color is unsurpassed, and fine examples can even 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. Opals displaying play of color are known as Precious Opals, and opals lacking play of color are known as Common Opals. Gems can be 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. Also important is Fire Opal, or Mexican Fire Opal, which is a transparent to translucent deep-orange red form of Common Opal. Fire Opal can also display play of color, and this is a rarity called 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. Some of these names have older sources, while some are recently coined trade names. Opal doublets, often used in jewelry, are thin slices of precious opal glued onto a base material. Such gems are considerably cheaper than solid opals, yet provide the same play of color. Opal doublets are sometimes coated with a thin layer or dome of clear Quartz to make them more resistant to scratches (since Opal is a relatively soft gem). These are sometimes called Opal triplets. A condition called crazing affects certain Opals, causing them to form internal cracks. Crazing is an interesting phenomenon, as it lacks consistency and is sometimes unpredictable. Although it can occur at random, its often takes place when an Opal removed from damp conditions is allowed to dry too quickly, or when an Opal is exposed to sudden intense light. Crazing may also take place when an Opal is subject to vibration, as during the cutting and polishing of a gemstone. The severity of the crazing and the time it takes to "craze" varies among gemstone. The origin is often a determining factor to its resistance to crazing, as some localities are less prone to crazing than others. A 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. Precious Opals are cut and polished into cabochons and used in all forms of jewelry, especially as pendants and ring centerpieces. Fire Opals are faceted into several gemstone cuts for jewelry. Boulder Opal is also a popular form which is used as jewelery, especially as cabochons. Opal, especially Common Opal, can also be carved into small ornamental figures.
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