The opal is usually divided into 3 groups:
noble opal, with its typical play of colors (harlequin)
fire opal, yellow-red
common opal, very widespread.
color: white, gray, blue, green, orange, black; sometimes it has harlequin.
Chemical Formula: (SiO2•nH2O),silicon oxide with water.
The typical feature of this gem is the harlequin, which consists of patches of light that change color as the angle of observation varies. The opal contains a variable percentage of water up to 30%. Since over time the water can evaporate, the opal can fracture and consequently the colors become less iridescent. The preservation of the opal in moist cotton preserves it from aging and enhances the play of colors. Opal is sensitive to pressure, shocks, acids and lye. Also pay attention to the embedding.
Noble opals are divided into 2 groups:
1) white opals: white or light background.
2) black opals: rarer, with dark gray, dark blue, dark green, black gray or black background (very rare)
The noble opal found on the mother rock (boulder opal) is called the opal matrix. Until the end of the nineteenth century, the best quality of noble opal came from the Andenisite lavas of Cervenica, in Czechoslovakia.
Subsequently, Australian deposits were discovered, of which the most famous (Lightning Ridge, White Cliffs, Coober Pedy, Andamooka, Bulla Creek and Burcoo River), in Brazil, Guatemala, Honduras, Japan, Nevada (USA).
The “cabochon” cut enhances the play of colors to the maximum. There are various mystifications: doublets, triplets, dyes, or impregnation with artificial resins. A synthesis of white opal has been produced in France since 1970
It is so called for its characteristic orange color. It rarely has harlequin (even if possible), it has a milky appearance. The excellent pieces are clear and are very sensitive to stress. The most famous deposits are: Mexico (Hidalgo and Queretero), Brazil, Guatemala, Honduras, USA, Western Australia.
Generally opaque it has many commercial names: Agate – opal, hyalite, milky opal, musk opal, prase-opal, chrysopal.