Opal - Encyclopedia

    Class : Silicate
    Subclass : Tectosilicate
    Crystal system : Amorphous/Disordered
    Chemistry : SiO2 . nH2O
    Rarity : Common


The opal is an amorphous silica gel or has a low disordered crystallization. Mostly made of silica microspheres (150 to 300 nm in diameter) visible only under the electron microscope and sometimes associated with disordered fibers of cristobalite and tridymite. In mineralogy, opals are classified into 3 families : opale-CT (silica gel + disordered fibers of cristobalite and tridymite), opal-C (silica gel + cristobalite fibers) and opal-A Silica gel only). The void between the microspheres are filled by air or water whose content can vary from 6% to more than 20%. The play-of-color (abusively called "fires") of certain opals results from the diffraction of the light through these layers of microspheres. The term "opal" comes from Sanskrit "upala" (precious stone) and then taken up by the Greek "opalios" and the Latin "opalus". It is generally found in volcanic and sedimentary environments. In volcanic areas, it is deposited at the hot water source and around geysers rich in silica (geyserite) or by circulation of hot siliceous waters affected by the fumarolic activity (gases). In sedimentary areas, it is formed by alteration of silicates and colloidal deposition (tiny particles of the size of the nanometer), in this case it sometimes happens to epigenize fossil organisms. Due to its partially totally amorphous nature, it never occurs in crystals, but in botryoidal or stalactitic masses, more rarely in veins. Finally, in a more anecdotal way, the opal constitutes the skeletons of some living beings : spicules of sponges, or shells of diatoms (siliceous algae) and radiolar (zooplankton) which by accumulation can give rocks (diatomite and radiolarite). The opal can be opaque to transparent and of varied colors resulting of many varieties:  hyalite (transparent and colorless), cacholong (milky white), resinite (yellow to brown), fire opal (red-orange and transparent) ; The precious opals present a play-of-color and are also classified according to their colors and transparencies : black opal (black with play-of-color), crystal (transparent with play-of-color), boulder (with matrix), koroit (thin veins in matrix with violent play-of-color), chocolate (brown with play-of-color). It is a mineral which has a great importance in jewelery, but can also be used in industry, radiolarites and diatomites are crushed to make abrasives, filters and insulating and absorbent products, they also enter in the preparation of dynamite. Finally, diatomite is also widely used in the cinema industry because of its very light weight.

Hyalite opal from Tarcal, Borsod-Abaúj-Zemplén, Hungary
Opal in matrix from Oregon, USA
1,31 ct faceted crystal opal from Welo, Ethiopia
44,12 ct pink opal cabochon from Peru

Opal in the World

Opal is a fairly common mineral, but deposits producing precious opals that can be used in jewelery are rare. The most famous are Australian deposits and have delivered extraordinary stones such as Coober Pedy, Andamooka, Bulla Creek, Burcoo River, White Cliffs, Lightning Ridge, etc... More recently the Welo region (Ethiopia) has produced many crystal opals and chocolates (picture on the right), which currently supplant Australian opals on the market. The most beautiful fire opals come from Mexico. The United States (Oregon) possesses some interesting deposits of opal as well as Slovakia, the latter was also the producer of the finest precious opals of the World before the discovery of the Australian deposits in 1923 then Ethiopian. Finally, the exceptional hyalite opals mostly come from Eastern Europe (Hungary and the Czech Republic).
2,08 ct fire opal from Mexico
0,89 ct fire opal from Mexico
0,88 ct fire opal from Mexico
1,79 ct fire opal from Mexico

Opal in France

France has some interesting opal deposits especially in Auvergne region correlate with the young volcanism : forcherite from St. Nectaire (yellow opal due to realgar and orpiment inclusions), resinite opal from Gergovie and the famous lussatite from Limagne. We also cano note the pink opal of Quincy curiosity in the Cher department locally named "quincyite" and whose color is due to the inclusions of colored sepiolites and organic pigments.

Special shapes

Opal can replace or mold the shape of some fossils : shells of bivalves and gastropods, bones, plants (petrified wood), belemnite rostres, ammonites, etc...

Fakes and scams

Due to its importance in jewelry, opals are usually treated but also synthesized, these practices are unfortunately not always disclosed on the market.

  • The synthetic opals : sometimes called Gilson or also Kyocera, are opals are made by compression, the stratification of the silica microspheres is forced and gives an unatural columnar play-of-color. Under the microscope, the batches of silica microspheres give on the surface of the stone a snakeskin texture absolutely absent on the natural counterparts.
Natural harlequin black opal
(very rare)
Opal with natural honeycomb patterns (rare)
Synthetic opal with columnar and snakeskin pattern
Snakeskin pattern typical of synthetic opals (visible under x10 magnification only)


  • Monarch opal : It is a synthetic opal released on the market in 2017, it looks much more like natural opal, but has characteristic black veins, rarely present on natural opals. Right picture showing a lot of these new synthetic opals usually sold around €10 per carat - Copyright © Stonephase.


  • Doublets or triplets : doublet if the cabochons are made of a thin layer of precious opal stuck on an opaque black base (basalt, glass, etc...) ; triplet if in addition the opal is covered with a layer of glass or colorless quartz.
Triplet seen from the side with quartz dome, thin layer of opal and black base
Triplet seen from the side with quartz dome, thin layer of opal and black base
Harlequin opal in triplet. Assembly difficult to see in view from above
Opal doublet (no dome on the surface)


  • Treated black opals : often called Andamooka black opals or "sugar and smoke" these opals dipped in sugar syrup and then carbonized with sulfuric acid.
Black opal treated, the treatment does not penetrate completely into the heart of the stone...
Black opal treated : before-after
Little penetrating treatment is always done on fairly thin slices
Thin slices of treated black opal ready to be cut.
Lot sold on eBay without any specification of treatment...

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  • Opalite : is found in tumbled stones, balls and beads for lithotherapy. It is just an industrial glass imitating opal, microscopic fluid texture, gas bubbles and refractive index different from the natural opal make it easy to make the difference.
  • Slocum opal : also known as "slocum stone", doublet or triplet made of metal particles covered with a glass or quartz dome. Can also be made of metallic sheets directly included in a glass. These stones can only deceive the neophytes, the visual aspect is very different from natural opal.
  • Crazing : natural phenomenon of rapid cracking (a few months) and fracturing of precious opals linked to dehydration. Some low-quality, low-cost precious opals with high porosity eventually suffer this phenomenon and break. In order to avoid unpleasant surprises, serious producers leave their rough opals in full sunlight a few months before cutting. It is impossible visually to know in advance if the opal pieces will undergo this phenomenon. To avoid : the black opals and chocolate opals from Ethiopia and Indonesia, the large pieces of rough coming from the countries considered as cutting centers (Brazil, Madagascar, India, etc...). Crazing can appear on every opal (ever the best quality) if it is heated.
Opalite sold without indication of the true nature of the material...
Opal slocum sold 190 € on ebay, expensive for a glass imitation...
Crazing typical on Ethiopian opals of low quality
Cracking of opal after cutrelated to dehydration (crazing)



Hardness : 5,5 to 6,5
Density : 1,9 to 2,3
Fracture : Irregular to conchoidal
Trace : White




TP : Opaque to transparent
IR : 1,40 to 1,46
Birefringence : -
Optical character : -
Pleochroism : None
Fluorescence : Yellow, green, blue


Solubility : Hydrofluoric acid

Magnetism : None
Radioactivity : None