Opal is one of the most beautiful gemstones. Its structure consists of the smallest spheres of silica (globules) and water between them. These gems are distinguished from the rest by a remarkable unique optical effect – opalescence. The transmitted light is divided into colored rays, as a result of which we observe an interesting play-of-color. Optical properties allow to observe different pure colors in the form of bright spots that can change their color depending on the angle of view and lighting in one sample. The color of opalescence depends on the size of the silica globules. With a size of about 0.1 microns, violet colors are obtained, with a size of 0.2 microns – red. The sizes between these values of the globules give all the remaining colors of the rainbow.
Depending of the structure, two types (types) of this mineraloid are distinguished: precious (with opalescence) and common (without opalescence).Precious opals, depending on the general perception of the base color of the stone, are divided into black (with a dark base) and white (with a light base). The opalescence picture, consisting of color spots of different sizes and shapes, is called a pattern. Such types of patterns are distinguished, for example: “honeycomb” – opalescence is represented by cells resembling a honeycomb; “Pinfire” – many small multi-colored dots; “Harlequin” (“Harlequin”) is the rarest pattern, opalescence in the form of colored spots, similar to a bright harlequin costume. There are more than a hundred color varieties. Stones with red and orange colors are the most valued. The larger quantity of multicolor spots are in a sample, the more costly a stone is. Opals from Australian mines are estimated higher than from other countries. Ethiopian opal is more common in the market, and its price is democratic.
DEPOSITS
The majority of precious opals comes from Australian and Ethiopian mines. Before 2008 year, Australia was a leader of opal mine, providing nearly 95% of all stones. Nowadays, Ethiopian deposits are the primary source. The first finds in Australia date back to 1849. Australian “white” and “black” opals come mainly from the following deposits: Lighting Ridge, Coober Pedy and Andamooka. Mexico is also an important mining region, it provides orange «fire» opals. Another deposits are the following: Brazil, The USA, Honduras, Russia and others. An interesting blue and pink color varieties without opalescence come from Peru. «Black» Australian opals with large rainbow spots are the most valuable.
Properties
Mineraloid:
opal
Chemical formula:
SiO2 * nH2O
Crystal Systems:
Amorphous
Mohs scale hardness:
5-6.5
Optical character:
isotropic
Cleavage:
none
Density:
2.15 (+0.08; -0.9) g / cm3
Luster:
Vitreous to resinous
Refractive index:
about 1.45; > 1.37 Opal Mexico
Birefringence:
None
Dispersion:
-
Color:
Any colour
Interesting facts
Price of the high-quality Australian samples may reach USD $ 10000 per carat.
The structure of opal contain up tp 20% of water.
There are pseudomorphoses of shells, bones of ancient animals, wood, due to the inherent ability of opals to replace materials of organogenic origin.
History
The first mention of this mineraloid dates back to Pliny the Elder in 77 AD. Opal as a gemstone has been known since ancient times in India and East Africa. In Assyria, Babylon, ancient Rome and Greece, stone came from the territory of modern Slovakia. A large stone adorned the crown of the Holy Roman Emperor Constantine. A necklace with Hungarian opals, made in the 16th century, is kept at the Hungarian National Museum in Budapest. This gem has been widely recognized since the end of the 19th century, when the development of deposits in Australia began. Today the best stones from Australia can be considered as investment. Synthetic (artificial) opals are known several decades, but they are not so beautiful and alive as natural stone.
Enhancement
Usually treatment is applied to highly porous opals, for example, of Ethiopian origin. Precious opals may react to changes in wetness and temperature. Over time, due to changes in the amount of water contained in them, they may crack and become cloudy. To prevent cracking, they can be stabilized by impregnating with various polymers. Impregnation, as well as oiling, can also visually hide existing cracks. Opals can be colored in almost any shade. And impregnation of light opal with sugar, followed by “carbonization” in sulfuric acid, usually darken the stone, and then its opalescence becomes more contrast and noticeable. For the same purpose, the rough material may be «smoked». Such smoke treatment is intended for low quality raw materials.
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