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. Gems 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. See the opal variety page for an extensive listing of all opal varieties.

Opal doublets, often used in jewelry, are thin slices of precious opal glued onto a matrix 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 (opal is a relatively soft gem). These are known as triplets.

USES
Precious opals (black and white) are cut and polished into cabochons and used in all forms of jewelry. Some forms of Mexican fire opal are faceted into several gemstone cuts, and others as cabochons. Opal matrix and boulder opal (layer of precious opal on top of, or embedded in, a reddish-brown rock called ironstone) has an attractive contrast that sparks interest, making it another form of opal used in jewelry.

VARIETIES
There are numerous variety for all different forms of opal. The opal variety page lists all varieties.

SIMILAR GEMSTONES
The play of color exhibited in opal is distinctive, and no gemstones (aside from some synthetic materials which somewhat emulate opal) can duplicate this gemstone. Mexican fire opal may appear similar to topaz, citrine, chrysoberyl, and carnelian.

ALSO SEE

• The mineral Opal
• Opal variety page

PICTURES

Special thanks to Gem Hut for most of our gem images. The gems displayed or others like them can be purchased through Gem Hut by clicking the link below each picture.

Additional opal images, both unpolished and cut, can be seen in the opal variety page.

Picture: Black Opal
Cut: Free-form cabochon
Size: 2.19 ct.
Collection: Gem Hut

Picture: White Opal
Cut: Free-form cabochon
Size: 4.49 ct.
Collection: Gem Hut

Picture: Jelly Opal
Cut: Cabochon
Size: 2.49 ct.
Collection: Gem Hut

Picture: Mexican Fire Opal
Cut: Cabochon
Size: 3.83 ct.
Collection: Gem Hut

Picture: Lemon Opal
Cut: Scissor , cushion
Size: 10.03 ct.
Collection: Gem Hut

Picture: Cherry Opal
Cut: Cabochon
Size: 2.04 ct.
Gem: GemHut

Picture: Precious Fire Opal
Cut: Cabochon
Size: 2.25 ct.
Collection: Gem Hut

Picture: Water Opal with bluish sheen
Cut: Cabochon
Size: Not measured (although picture is actual size)
Collection: Hershel Friedman

• Many more images of opal, in both unpolished and cut forms, can be seen in the opal variety page.