other names: /
mineral class: Oxides and hydroxides
chemical formula: SiO2 • H2O
Chemical elements: Silicon, oxygen, hydrogen
Similar minerals: ?
colour: multi-colored (including black, white, red, yellow, green)
crystal structure: /
mass density: 2,0
magnetism: not magnetic
Mohs hardness: 6
stroke color: White
transparency: translucent to opaque
General information about the Opal:
Of the opal describes a mineral, which is counted among the oxides and hydroxides and can have a water content of up to twenty percent. Opals are made of silica (silica gel) in the form of small beads. In the interstices of these beads, the incoming light is reflected and leads to a spotty rainbow play of colors that dazzles in innumerable tones. Their eye-catching iridescence, which is also called opalizing, makes these stones, which are used exclusively in the jewelry industry, unmistakable. Depending on their color and appearance, they are subdivided into three groups, namely the common opals, the fire opals and the noble opals.
The name of the mineral means translated as precious stone, originally from Sanskrit and was taken with the Latin "opalus" and the ancient Greek "opallius" in the European languages. While fire opals usually glow bright red, but often also shimmer orange or yellow, the Opal is almost transparent and shows only an inconspicuous play of colors. Opaline on the other hand inspire with different attractive color variants and are subdivided according to their appearance into several subspecies.
Origin, occurrence and localities:
Opals are produced exclusively hydrothermally, but can develop in volcanic rocks as well as in tuff rocks, in organic matter or in sediments. A socialization with chalcedony occurs very often. Opals are solids that do not form crystals, but in the cavities of various types of rocks drop-shaped or massive structures, tubers, crusts or veins. The starting material represents a silica solution in the rock, whose water content gradually decreases with time. As a result, the silica is separated in the form of globules, which rearrange.
In the past, Slovakia was considered the country with the richest abundance of particularly beautiful specimens. Even the Romans imported their stones mainly from Slovakia. Today Opals are also being promoted in Russia, Brazil, Ethiopia, South Australia, Mexico, Japan and Honduras.
History and usage:
Even in ancient times, opals were extremely sought after as gemstones and were specifically promoted in Central and Eastern Europe by the Romans. Even in South America, the dazzling stones were highly prized by indigenous peoples such as the Maya or Aztecs. In Europe, the Opal gradually fell into oblivion with the fall of the Roman Empire and did not experience a revival until the second half of the 19th century. It is thanks to the artists of Art Nouveau, who discovered the stone for themselves, that pieces of jewelery with opals still enjoy great popularity to this day.
As gems particularly sought after are those specimens that are assigned to the group of precious opals. Above all, the white opal, on whose white base color countless colors iridescent, is in Cabochon cut very well. Just as popular are jewelery with black opals and boulder opals, because the colorful colors shimmer on such stones against a dark background. Other noble opals, which are often processed into precious pieces of jewelry, are the Harlequin Opal and the Crystal Opal with red light reflections against a white background.