Opal is Beautiful
If ever there was a most beautiful gemstone contest I believe the opal would win hands down. It is like the sunset and sunrise wrapped up in a volcano.
From the first time I laid eyes on opal I have been enthralled by the beauty of the stone. There is no question that the gemstone is a sight to behold. But I stand corrected in that opal is not a gemstone in the true sense of the word. The four true gemstones are diamond, sapphire, ruby and emerald and they are crystals.
Opal is a hydrated amorphous form of silica and may often have a water content as much as 10%. It is classed as a mineraloid along with glass, jet, obsidian and pearl.
In the article, “Are Cultured Pearls Real?” I described how pearls are coated with layers of very small, little bricks of nacre. Similarly, opal is made up of microscopic layers of little spheres and it is with the way light reflects its way along these layers that creates the spectacular colors we see. Precious opal ranges from clear through white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black. Of these hues, the black opals are the rarest, whereas white and greens are the most common.
Singles, Doublets and Triplets
Generally opals are cut and polished in the cabochon shape and can be in singles, doublets or triplets. The singles will be comprised of the solid stone from front to back. A doublet may be made by cementing a layer of a dark stone to the back of the single stone to enhance the color of the stone, most commonly ironstone, dark or black common opal (potch), onyx, or obsidian. The beauty of opal is everywhere.
The darker backing emphasizes the play of color and results in a more attractive display than a lighter potch. An opal triplet is similar to a doublet but has a third layer, a domed cap of clear quartz or plastic on the top. The cap acts as a protective layer for the opal. The top layer also acts as a magnifier to emphasize the play of color of the opal beneath, which is often of lower quality. Triplet opals have a more artificial appearance and are not classed as precious opal. These changes are made to bring out the beauty of opal.
More Types of Opal
Along with the gemstone varieties of opal that show a play of color, many other kinds of common opal include the milk opal, milky bluish to greenish (which can sometimes be of gemstone quality); resin opal, that appears as honey-yellow with a resinous luster; and wood opal, which is caused by the replacement of the organic material in wood with opal. This change can take place over millions of years.
Mexico produces a wonderful array of fire opals. Fire opal is a transparent to translucent opal with warm, body colors from yellow to orange to brilliant red. Although it does not usually show any play of color, occasionally a stone will exhibit bright green flashes. Mexican opals are sometimes cut in their natural rhyolitic host material if it is a hard enough quality to allow cutting and polishing. This type of Mexican opal is referred to as a Cantera opal. Mexican water opal often is a colorless opal which exhibits either a bluish or golden internal sheen to the stone.
Girasol opal is a term that may be mistakenly and improperly used to refer to some fire opals, as well as a type of transparent type milky quartz from Madagascar which sometimes displays an asterism, or star effect, when cut properly. However, the true girasol opal is a type of hyalite opal that often will exhibit a bluish glow or sheen that follows the light source around. It is not a play of color as seen in precious opal, but rather an effect from microscopic inclusions. It is also sometimes referred to as water opal too when it is from Mexico. The two most notable locations where this type of opal can be found are Oregon and Mexico. The beauty of opal may be found everywhere.
Australian opal has often been reported as accounting for 95 to 97% of the world’s supply of precious opal, with the state of South Australia accounting for almost 80% of the world’s supply. Ethiopian opal has been making inroads in world production in the last while but due to the different methods of weighing, it is difficult to determine the proper figures. The opal, found mostly in the form of nodules, is of volcanic origin and is found predominantly within weathered layers of rhyolite. Rhyolites that cool too quickly to grow crystals form a natural glass or vitrophyre, also called obsidian.
Black Fire Opal
The Virgin Valley opal fields in Humboldt County in northern Nevada produce a great variety of precious black, white, fire, crystal and lemon opal. The black fire opal is the official gemstone of Nevada. Most of the precious opal is partial wood replacement which occurs over eons of time. The precious opal is hosted and found within a subsurface horizon or zone of bentonite in place which is considered a lode deposit. Opals which have weathered out of the in-place deposits are alluvial and considered placer deposits. Other Miocene-age opalised teeth, fish, bones and a snake head have been found. The beauty of opal is in its fire but there are other colors which are highly sought after.