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www.mineralgallery.co.za Minerals K-O |
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Opal |
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Name
Opal
Chemistry
SiO2-nH2O; Hydrated Silicon Dioxide
Uses
It is used as gemstones when cut en cabachon and the one variety Mexican Opal is often faceted
Color
Colorless or white, also blue, green, yellow and red shades and black, it can have flashes of color when turned, this property is called opalescent
Hardness
5-6
Specific gravity
2-2.5 very light
Crystals
It is amorphous which means that it lacks a crystal structure, It occurs as a massive and as nodules and it fills cavities as well as a replacement for ammonites or wood
Accompanied by:
Calcite, quartz, fluorite, zinc, silver, iron and nickel
Fracture
Conchoidal
Luster
Vitreous to pearly
Cleavage
No cleavage
Streak
White
Similar to:
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
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.
Opal has been a popular gem for many centuries and has a very interesting structure.
Opal is considered a mineraloid because this structure is not truly crystalline. The chemistry of Opal is primarily SiO2 and varying amounts of water. The amount of water varies from 5 -10% and greater.
This water can help geologists determine the temperature of the host rock at the time the opal formed.
Although there is no crystal structure, (meaning a regular arrangement of atoms) Opal does possess a structure nonetheless. Random chains of silicon and oxygen are packed into extraordinarily tiny spheres.
These spheres in most Opals are irregular in size and inconsistent in concentration.
Yet in Precious Opal, the variety used most often in jewelry, there are many organized pockets of the spheres. These pockets contain spheres of approximately equal size and have a regular concentration, or structure, of the spheres.
This has the effect of diffracting light at various wavelengths, creating colors. Each pocket produces a different color, with a different intensity depending on the angle from which a viewer sees it.
The multicolored flashes of light that Opal emits gives it a truly beautiful and valuable look.
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 polishedith a substantially reduced risk of crazing.
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.
Much information on this page was generously contributed by Allan W. Eckert, the author of the most exhaustive and detailed book on opals. His book, The World of Opals (John Wiley & Sons, NYC, 1997), was extensively reviewed in Lapidary Journal Magazine, which said "A book that all opal lovers have been waiting for...this is one of the most complete books that has ever been published for any gemstone
