OPAL - Precious, Fire and Common
Opals are composed of a desiccated silica gel that, upon drying, forms spheres on a microscopic structural level.  The size, roundness and space between these spheres dictate the ultimate quality of play of color that appears in each individual stone.  This mysterious play of color can be of one or many hues, and can occur in countless patterns on the surface of and within the stone.  There are three types of opal:  precious or noble opal, fire or Mexican opal, and common opal.

Precious Opal

Precious opal is what most people think of when visualizing opal.  In the opal trade most precious opals are classified as either white or black opal, depending upon the depth of color in the body of the stone.  Precious opal has a transparent to opaque colorless, white, yellow, brown, gray or black body color; superimposed on this body color are rainbow-like flashes of color that can change color as the viewing direction of the observer changes.  To best display this play of color phenomena, precious opals are usually cut as round or oval cabochons (cab-o-shawns).  A cabochon cut opal has a smooth polished convex surface and a flat to concave bottom.  The play of color can occur as pinpoints of light or broad flashes of color and anything in between; the texture, type, intensity, consistency, and color of these flashes of light dictate the value of the opal.
Determining value in precious opal depends on several factors.  Black Opals are more valuable than White Opal.  An opaque stone is valued less than a translucent or semi-transparent stone.  Color, size and location of the play of color in an individual stone affect the value greatly.  Red and orange flashes of color are rarer and more valuable than blue and green flashes of color.  An “harlequin,” or mosaic patchwork-like pattern in large patches over the stone, is preferred over a “pinpoint” pattern. A stone exhibiting dead spots in the middle will have a lower value.   
Because opals tend to occur in very thin seams in surrounding sandstone matrix, ironstone matrix and limonite, opals are often cut into doublets and constructed into triplets to protect the surface of the thin opal layer which is 5½ to 6½ on the Mohs’ scale of hardness.  A natural doublet consists of a layer of opal backed by the rock originally surrounding it.  This adds stability to the layer of opal.  Solid opals tend to be more valuable, but beautiful doublets and triplets can be made from what otherwise could not be set into jewelry because of the inherent fragile nature of the thin seam of opal.  A man-made doublet consists of a layer of opal glued or cemented usually to a black onyx backing to add durability to the opal itself.  All triplets are man-made and consist of a layer of opal sandwiched between a black onyx back and a rock crystal cap.  This is another way to preserve and magnify the layer of precious opal which otherwise would be considered too fragile to set as jewelry.  Opals should never be subjected to a steam or ultra-sonic cleaner.  They should be wiped with a dry clean soft cloth. 
Precious opal is mined almost exclusively in Australia, but small deposits also occur in Brazil, Honduras, Guatemala, Indonesia, Japan, Mexico, Russia, and the United States (Nevada, Idaho and California).  Before 1868, opal was only known to occur in present day Eastern Slovakia and generally was of a poor quality.  By 1868, however, the discovery of vast deposits of precious opal in qualities never seen before in various parts of the Australian continent introduced opal to the world at large.  Historic and active opal mining sites occur at Lightning Ridge (the primary source of black opal), White Cliffs in New South Wales, Coober Pedy, Mintabie and Andamooka in South Australia, and numerous deposits in Queensland Australia. 
Trade terms referring to different types of opals can be confusing as many have been applied improperly.  Below find some of the more commonly encountered terms for different forms of opal;
Black Opal - Transparency: transparent, translucent or opaque. Body color:  black, gray, brown, blue or otherwise dark body color, with play of color.  Red and orange play of color is more valuable than violet, blue or green play of color.
Crystal Opal - Transparent body color with strong play of color.  When transparent near colorless or light colored it is just called crystal opal; when the transparent body color is black, gray, or dark in tone it is termed black crystal opal.
White Opal - Transparency: transparent, translucent or opaque. Body color:  white, off-white or cream or otherwise light body color with play of color. Red and orange play of color is more valuable than violet, blue or green play of color.
Boulder Opal - Precious opal surrounded or backed by brown limonite sandstone matrix.  When precious opal is banded or dotted in matrix it is called matrix opal.  Ideally, a predominance of opal over matrix results in a more valuable stone.
Jelly Opal - Near colorless transparent body color with play of color.  A more intense play of color in red or orange, as opposed to violet, blue or green is more desirable and results in a more valuable stone.
Harlequin Opal - refers to the pattern of the play of color when it occurs in large mosaic-like patches.  Considered to be the most desirable and valuable opal.  Red and orange play of color is considered more desirable than violet, blue or green.  In the best quality harlequin opals the patches are close together and no dead or dark spots are apparent.
Fire or Mexican Opal
Fire or Mexican opal is descriptive of the intense and saturated body color of this type of opal, which occurs in Mexico, Guatemala, Brazil, the United States and Western Australia.  The color is predominantly Yellow, Orange, Red or any combination of these colors and is only transparent in the best qualities.  When transparent, the opal is often faceted and in this form can sometimes be mistaken for citrine or garnet.  Fire opal with play of color does exits, but there is often little or no play of color.      
Common Opal
Common opal is also known as potch to mineralogists.  This form of opal is opaque or semi-translucent and does not show any play of color.  It has a somewhat flat, waxy appearance and can occur in any color of the rainbow.  A pink and lovely greenish-blue translucent common opal is mined in Peru and is often fashioned into beads. 
Opal Properties
Species:  Opal
Varieties:  White Opal, Black Opal, Boulder Opal, Crystal Opal, Jelly Opal, Harlequin Opal, Fire or Mexican Opal, Common Opal.
Chemical Composition:  SiO2_nH2O - hydrous silicon dioxide
Color:  White, Cream, Yellow, Orange, Red, Blue, Green, Gray, Brown, Black.
Hardness:  5 ½ to 6 ½
Crystal System:  Amorphous
Refractive Index:  1.45 (+.20, -.80)
Specific Gravity:  2.15 (+.08, -.90) 

Images by Oscar Heyman.
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