A opal is a Minéral composed of Silice hydrated formula SiO2, nH2O.
The opal, transparent with translucent, generally amorphous, consists of a more or less regular stacking of silica balls. The crystals are formed only in Pseudomorphose, most frequent being those of wood, the shells and the bones.
The opal is a hydrated form of silica whose crystalline nature was highlighted only by X-rays diffraction. One admits a structure “arranged” in spheres of silica of 150 Nm diameter which follow one another with intervals of 4,1.10-10 m and in more or less regular layers. The quantity of water contained in the opal varies its density which remains close to 2. The opal is thus not very compact and the introduction of pigments makes change its color. The pure mineral is transparent or milky, if it contains oxides iron, for example, the color varies yellow with brown or even with orange but the coloring of the opal is due, especially, with the plays of light caused by its stratification which cause Interférence S and give this pearly glare.
The fires, therefore quality, of the opals (iridescence) are due to the diffraction of the Lumière by layers of perfectly arranged balls of Silice, (the blue color of the butterflies Morpho is due to the same phenomenon of Diffraction by regularly spaced microscopic structures). This diffraction does not exist in the common opal called " potch" , in which the spheres, smaller and of sizes different from/to each other are laid out randomly. Reflections white, blue, gray and black (rarer thus the stones are more expensive!) are rather frequent but the red reflections due to larger spheres (350 Nm) are very rare…
As for the Opalescence, it relates to the optical phenomenon observed on the translucent bottom of many opals.
The opal presents various varieties, of which some are used in Joaillerie and are classified like fine stones S; these varieties are characterized by their origin, the fill color or its nature. Without plays of color it is classified " commune" , with plays of color it is classified " précieuse". The " drawing arlequin" indicate a distribution extrèmement rare of the colors of an opal, that of the dress to the well defined squares, it is not a variety. An opal " arlequin" can be black, white, boulder….
- the white opal , or noble opal , with sharp fires of frank colors on translucent white zone with milky;
- the black opal , with multicoloured sharp fires on bottom sinks gray with black; ; the black opal comes almost exclusively from Lightning Ridge - the spectral colors appear more intense on the dark bottom. The stones of high-quality are extremely rare.
- the opal " crystal" , transparent, with or without plays of color;
- the opal of fire, transparent, yellow orange or red, with or without plays of color; ;
- the opal " boulder" , Australian, formed in the cracks and vacuums of a rock host, a sandstone rich in iron (ironstone); it is exploited mainly with the Queensland.
- the opal matrix , opal containing of the fragments of the rock in which it was formed; it is practically exclusive with the area of Andamooka, in southernmost Australia. Porous, rather common and, consequently, with less great value, the matrix is often soaked with sugar and pulp in the sulphuric Acid concentrated, which causes the deposit of the carbon of sugar in the pores of the stone, giving him a black…
- cacholong , white-bluish color or cream, having the aspect of the porcelain, without fires (the term also applies to a variety of Calcédoine);
- the opal hyalite , vitreous (colorless) and without fires;
- the hydrophanous , opal whose transparency and fires appear only if it wet or is immersed in water.
Layers
The Lessivage due to the erosion involves silica in solution, to the tablecloth. This scrubbing, tertiary sector relates to as well the rock as the shells taken in the deposit. The opal settled, by packages in the rock or in the place of the shells, when the tablecloth is descended following one long period from dryness.The deposit is thus not foreseeable, there are no veins, and the exploitation must be done very delicately, with the hand or with small machines (more or less artisanal exploitation) not to miss the deposits which can be large like the fist or even less.
The principal layers are in Australia
First discovered opal in Australia goes back to 1849 and was made by the German geologist Johannes Menge. The production started with White Cliffs (NSW) in 1890 and with Opalton (Qld) in 1896.
The Stuart Range Opal Field was discovered during the prospection for gold in 1915. The fields of opal of Coober Pedy (photo opposite) produce the ¾ opals of the world. They extend on 70 km from Mintabie in north with Andamooka in the south, discovered between 1920 and 30 and, whose opals have a low water content (< 6%) and bright colors.
The opal is in sandy layers of clay deposited with the Cretaceous by the not very deep sea which recovered the three broad depressions which form the Great Artesian Basin : Carpentaria Basin, Eromanga Basin and Surat Basin.
Lightning Ridge is known for these skeletons transformed into opal of which Pandora which comes from the shoulder of a plésiosaure.
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with the Mexico: San Juan LED Rio discovered in 1855. The photograph opposite watch the color of " Red fire" a Mexican opal.
Largest opals
The largest opal gemmeous quality in the world weighs 4,72 kg
The Jupiter Five : 5 kg, the largest piece of opal not worked
Persephone: polished opal.
Artificial opals
One can give an opalescent aspect to a transparent material by forming small spangles or bubbles, which will create the same phenomenon of diffraction (see the article Diffraction pattern ).The chemical reaction of Stöber makes it possible to produce suspensions of silica balls of a diameter of a few hundreds of nanometers. These suspensions give opals by simple decantation followed by a heating to 945°C during a few hours. The opals obtained are milky.
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In the Glassmaking, one can use a Verre containing Fluor. At the time of the Solidification, it will be formed Précipité S of Fluorure of sodium (NaF) and Fluorine (CaF2). These particles make less than 0,5 µm of diameter, and typically account for 5 to 7% of total volume.
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After the closing of Gilson which was the pioneer, the only producers of synthetic opal are Kyocera d' Inamori and Chatham Created Gems of San Francisco. The synthetic opal present of the larger fields of color, on irregular board and of the colors plus sharp, but it is not always obvious. One sees there also layers by transparency, which do not exist in the natural opals.
Symbolic system
The Noces of opal symbolize the 21 years of Mariage in the French folklore.
Legends of the Opal
The origin of the opal term would come from the word Sanskrit “ upala ” meaning invaluable stone. Time of the Greeks and Romans she was extremely appreciated on the one hand like amulet, and on the other hand for her therapeutic virtues in close relationships with the eye.
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Pline tells that Nonius preferred to leave Rome and its richness, sacrificing its load and its fortune, to yield its opal, of the size of a nut, with Marc-Antoine which undoubtedly would have liked to offer most beautiful of the gems in pledge of love to Cléopâtre.
- a Hindu legend tells that the Eternal changed into magic cloud a woman who was coveted by three gods. The first Brahma, made him gift of the color of the azure in order to be able to distinguish it from the other clouds; the second, Shiva tinted it rouge ; as for the third, Vishnou, it para of the splendor of the sun. The Eternal then brought back it on ground in the shape of an opal.
See too
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