How are opals found


A relatively common mineral from the class of oxides and hydroxides.
Opal is one of the few amorphous minerals, which means that it has no crystal structure.

In addition to the amorphous shape, the opal has a few other special features.
Opal is made up of tiny silica beads. If these are approximately the same size and in the correct arrangement, diffraction and interference can create the desired play of colors (Opalize) produce.

Opal in the shop

In addition, Opal contains up to 20% water, which can lead to problems if improperly handled and stored (see below: Useful information).
Transparent to translucent ones often appear orange and bluish in transmitted light when viewed normal to transmitted light. This phenomenon will opalescence called

In the gemstone and jewelry trade, a distinction is made between three types, precious opal (with play of colors), common opal (without play of colors) and fire opal. The latter occurs with and without a play of colors.

Mexican fire opal with and without play of colors

Origin of name: The term opal came from Latin opalus or the Greek ὀπάλλιος opal for 'precious stone' and probably comes from the Sanskrit word upala.
Source: Wikipedia

Synonyms and trade names: There are a large number of trade names that refer to color, play of colors, type of play of colors, inclusions (moss opal) or the origin.
Some of the best known are black opal, milk opal, crystal opal, boulder opal, matrix opal, harlequin opal, pinfire opal and many more.

Mexican black opal from an old find (1980s)
The left stone developed a crack and was therefore processed into a doublet (opal on onyx)

Black opal from Lightning Ridge, Australia

Milky opal from Coober Peedy, Australia

Two Australian boulder opals

In order to be able to utilize very thin opal layers, these become Opal doublets and Opal triplets processed. In the case of duplicates, the opal layer is glued to a mostly black carrier (onyx or plastic). In the case of triplets, there is also a protective, colorless top made of mostly glass or plastic, occasionally rock crystal.

Opal doublet and triplet

Possible confusion: Common opal, i.e. opal without a play of colors, comes in many colors and patterns and can easily be confused with other minerals. There are yellow to brown "honey opals" that are similar to amber. There are banded opals that look like jasper or moss opals that show dendrite inclusions similar to moss agates.

Jasper-like common opal

Precious opals with a play of colors can be confused with synthetic opals.

Synthetic opals

Porous rock with inclusions of precious opal is found in Andamooka, South Australia. Often times, this type of opal known as the Andamooka Matrix is ​​color enhanced to mimic expensive black opal. To do this, the stone is soaked in a sugar-containing solution. Subsequent treatment with sulfuric acid converts the sugar into black carbon.

There are also glasses that show opal-like colors (opal glass, slocum glass).

Occurrence: Opal is found on all continents and in Antarctica. Most of the sites, however, only deliver common opal.
The most important and best-known deposits for precious opal are in Australia. The coveted black opal comes from Lightnin Ridge, New South Wales. The Yowah opal field in Queens mainly supplies boulder opals and Coober Peedy in South Australia is known for fine crystal and milk opals.

Mexico is famous for fine fire opals with and without play of colors, as well as for so-called matrix opals. The matrix opal is a host rock (matrix) made of light to dark brown iron stone, the cavities of which are filled with opalescent precious opal.

Some very large fire opals without any play of colors come from Brazil. The color is mostly yellow to orange, but occasionally intense orange-red stones are found that need not fear comparison with the best Mexican fire opals.

Brazilian fire opals
The largest stone weighs 48.20cts

Opal cat eyes are also found in Brazil:

The Afar Province in Ethiopia has been an important source of opal since the 1990s. The stones are known in the trade as Welo Opals, after the historical province of Welo (also Wello or Wollo).

Two very nice Welo opals

Processing: Opals are very sensitive to heat and must be gripped when soldering. Do not use lightbulbs or halogen spots in the showcase. Strong fluctuations in temperature and humidity must be avoided at all costs.

Useful information: the high water content of opal (up to 20%!) is a problem that should not be underestimated. If the opal releases this water, this can range from a reduction in the luminosity of the colors, cloudiness, the formation of cracks and cracks to the destruction of the Steins lead.
Some opal experts advise storing opals and opal jewelry in water.
Others believe that once the opal has acclimatized, this only leads to new tensions.

In any case, there is agreement that opals must be kept away from heat and large fluctuations in temperature and humidity must be avoided.

Opal in the shop


Gemmological properties of opal