Opal is classed a mineraloid, crystalline forms of silica, which are classed as minerals, unlike and hydrated amorphous form of silica (SiO2·nH2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. It is colorless when pure, the vast majority of common opal occurs in opaque, dull yellows and reds. It varies from essentially amorphous to partially crystalline. Precious opal is the least crystalline form of the mineral, consisting of a regular arrangement of tiny, transparent, silica spheres. Regularly arranged spheres of a particular size create a diffraction effect called color play. It is widespread and is deposited at low temperatures (up to 400°F/200°C) from silica-bearing, circulating waters. It is found as nodules, stalactitic masses, veinlets, and encrustations in most kinds of rocks. Opal constitutes important parts of many sedimentary accumulations, such as diatomaceous earth. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl, and basalt. It is the national gemstone of Australia

Name: Known since antiquity, derives its name from the Roman word opalus, which means “precious stone.”

Association: Chalcedony, quartz, calcite, topaz, goethite, cinnabar, phillipsite, magnesite, fluorite, ikaite

Polymorphism & Series: The structure of low-pressure polymorphs of anhydrous silica consist of frameworks of fully corner bonded tetrahedra of SiO4. The higher temperature polymorphs of silica cristobalite and tridymite are frequently the first to crystallize from amorphous anhydrous silica, and the local structures of microcrystalline also appear to be closer to that of cristobalite and tridymite than to quartz. The structures of tridymite and cristobalite are closely related and can be described as hexagonal and cubic close-packed layers. It is therefore possible to have intermediate structures in which the layers are not regularly stacked.

Mineral Group: Mineraloid

Chemical Properties

Chemical Classification Silicates; Tectosilicates; Silica Group
Formula SiO2 · nH2O

Opal Physical Properties

Color Colourless, white, yellow, red, orange, green, brown, black, blue
Streak White
Luster Vitreous, Waxy, Greasy, Dull
Cleavage None Observed
Diaphaneity Transparent, Translucent
Mohs Hardness 5,6 – 6
Crystal System Does not apply because opal is amorphous.
Fracture Conchoidal
Density 1.9 – 2.3 g/cm3 (Measured)  
Habits Massive, cavity-fillings such as in fractures and geodes, nodular, reniform or as a replacement of other minerals and wood.
Other Characteristics Most specimens will fluoresce white or pale green, some phosphoresce and all specimens can be very sensitive to impacts and low temperatures.
Associated rocks Chert (a form of microcrystalline quartz), volcanic rocks and many others.

Opal Optical Properties

Type Isotropic
Color / Pleochroism Non-pleochroic
RI values: nα = 1.400 – 1.460
Birefringence Opal-AG and Opal-AG are optically isotropic but may show anomalous birefringence due to strain. The microcrystalline varieties show birefringence: opal-C is length-fast, opal-CT is length-slow, but almost isotropic.Isotropic minerals have no birefringence
Relief Moderate

Opal Occurrence

Opal is a mineraloid that accumulates at a relatively low temperature and can occur in almost all kinds of rock fissures, most commonly found with limonite, sandstone, rhyolite and basalt.

Australia produces about 97 percent of the world production. About 90 percent of this is called light or white and crystal opal. White makes up 60 percent and all areas produce white ; crystalline opal or pure hydrated silica constitutes 30 percent; 8 percent black and only two percent rock . The town of Coober Pedy in South Australia (see below) is a major source of opal. Another Australian town in New South Wales, Lightning Ridge is a main source of black opal – with a mainly dark background (dark-gray to blue-black, displaying a colorful gameplay).

Fire opals are mostly found in Mexico and Mesoamerica. In Honduras, quality black opal is extracted from volcanic ash deposits. It is known for its opal stability. There’s opal in South America. The Brazilian city of Pedro II produced the opal discovered in 1930.

Virgin Valley Opal Fields, north of Nevada, produces black, crystal, white and fire opaque. The most valuable opals are wood. The mines discovered in 1904 produce large quantities of jewelry for hundreds of season visitors. The three mines give the public the opportunity to open the jewels for themselves for a fee. The largest black opal in the Smithsonian Museum comes from these mines.

Another source of white, in the United States is Spencer, Idaho. A high percentage of opals occurs in the film. As a result, most of the production enters the production of twins and triplets.

Opal Varieties

Opal Carving of a Walrus Boulder: Boulder opal consists of concrete and fracture fillings in the matrix of dark siliceous irons. It is sporadically found in western Queensland, Australia, from Kynuna in the north to the Yowah and Choroid opal fields in the south.

Black opal contains a predominantly dark background (dark-gray to blue-black)

Fire opal is a translucent to semi-opaque stone, usually yellow to bright orange and sometimes almost red. It shows pleochroism from certain angles (the ability to show different colors when viewed from different directions under transmitted light).

Peruvian opal (also known as blue opal) is an opaque, semi-opaque blue-green stone found in Peru. The matrix is ​​often cut to include more opaque stones. Pleochroism does not show.

Synthetic opal, as it occurs naturally, opals of all varieties are synthesized experimentally and commercially. The discovery of the precious opaline sequential sphere structure led to its synthesis in 1974 by Pierre Gilson. The resulting material can be distinguished from normal opalities and natural opals; under magnification, it is seen that the color patches are arranged in a “lizard skin” or “chicken wire” arrangement. Synthetics are further distinguished from natural substances by lack of fluorescence under the ultraviolet light of the former. Synthetics are also generally lower in density and generally very porous; some may even stick to the language.

Two notable synthetic manufacturers are Japan’s Kyocera and Inamori. However, since most synthetic synthetics contain substances (such as plastic stabilizers) not found in the natural opal, they are referred to as more accurate imitations. The imitation opals seen in vintage jewelry are “Slocum Stone”, which is usually made of laminated glass interspersed with foil pieces.

Cultural Importance

  • It is widely used in jewelry. The color play of some opals makes them popular in the “mood rings halk, which are expected to reflect the emotional state of the user.
  • It believes that some have healing power. Reported powers include alleviating depression and the ability to help the user find true love.
  • It is thought to improve the positive qualities of people born in the Zodiac. Black opal is recommended for those born in Scorpio, and Boulder opal brings a good chance for those born in Aries.
  • Its was particularly popular during the Art Deco era, and gemstone artists preferred them over all other stones.
  • It is the official gemstone of the South Australian and Australian Partnership and is called the Australian Women’s National Basketball Team Opals.
  • It is the official birthstone of October.
  • The Nevada state stone is a precious black opal located in the Virgin Valley, Humboldt County, Nevada.
  • Opal is the name of a popular woman.

Opal Formation

Sometimes, when conditions are ideal, silica spheres are formed in the silica-rich solutions in the world and settle in a space under gravity to form layers of silica spheres. The solution is believed to have a precipitation rate of about one centimeter thick over five million years at a depth of forty meters. If the process allows the spheres to reach a homogeneous size, valuable opal begins to form. The sphere size for the valuable opal is from about 150 to 400 nanometers which produce color gamut with diffraction in the visible light range of 400 to 700 nanometers.

Each local opal area or formation should contain some kind of cavity or porosity to provide a site for opal deposition. In volcanic rocks and adjacent environments, opal can only fill cavities and cracks, while sedimentary rocks have various cavities created by the decomposition process. Separation of carbonates from rocks, nodules, many different fossils, existing cracks, open ironstone nodule centers and horizontal sutures provides numerous molds ready for deposition of secondary minerals such as opal.

Almost all economic production comes from sedimentary deposited deposits associated with the Greater Australian Basin, while Australia has volcanic hosted and other valuable opal species. Australia has three valuable opal species that host three main natural sediments, black opal from Lightning Ridge in New South Wales, white opal from South Australia, and Queensland rock and matrix opal.

Uses of Opal

Opals are a silica oxide (a combination of oxygen and silica) and are one of the five main gems considered precious stones.

These five jewels are diamonds, rubies, emeralds, sapphires and opal. The primary use of opaline is exhibited as jewelery.

Ninety-five percent of the opals are used for jewelery, ornament and collector’s market in precious stone applications. These opals are often rewarded for their colors and designs. Some people use as part of gemstone therapy or as a focal point during incantation practices. As an example, fever opal is often used with para rituals. It , sometimes called the queen of jewelry, has been respected for centuries. The Romans believed that the opals represented purity and hope; The Greeks thought they had given their owners their predictions. They were believed to be the talisman of the gods, to protect themselves from disease and to protect their owners from harm.


  • Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
  • Mindat.org. (2019). Opal: Mineral information, data and localities.. [online] Available at: https://www.mindat.org/ [Accessed. 2019].
  • Wikipedia contributors. (2019, June 13). Opal. In Wikipedia, The Free Encyclopedia. Retrieved 00:37, June 20, 2019, from https://en.wikipedia.org/w/index.php?title=Opal&oldid=901668208
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