About Opal | Opal Classification | Varieties of Natural Opals
Body Tone
| Transparency | Formation of Opal
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About Opal

Australia produces over 90% of the world's supply of precious opal, and probably all of the highest quality gems. The ex-mine value of Australia's opal output was estimated at over $A100 million in 1991, and more than half came from New South Wales in eastern Australia. The major opal producing areas in Australia are Lightning Ridge and White Cliffs in New South Wales; Coober Pedy, Mintabie and Andamooka in South Australia, and localities producing boulder opal in western Queensland. Over 99% of the most valuable black opal comes from only two localities: Lightning Ridge and Mintabie. Around $30 million worth of opals was produced from Lightning Ridge in 2005/06. On April 10, 2008 NSW Premier Morris Iemma announced the government would designate the black opal as the state's official gemstone.

The New South Wales opal fields are all situated in arid inland areas, on ridges surrounded by black soil plains. The sedimentary opal deposits occur in deeply weathered Cretaceous rocks formed 65 to 135 million years ago within the Great Australian Basin. The opal is found at shallow depths in sedimentary rocks through which there has been considerable silica seepage into cavities and other structures. Gradual loss of water from the silica gel has resulted in hardening of the material and the formation of opal.

Host rocks contained a variety of voids formed by the weathering process, which leached carbonate from boulders, nodules, fossils, cracks, hollow centres of ironstone nodules and horizontal seams. Most opaline silica deposited is common opal (or potch). It does not show a play of colour. Opal also fills pore space in sand-size sediments, cementing the grains to form deposits known as matrix or opalised sandstone. Opal is often associated with lineaments or faults which break the rock, providing conduits for the movement of ground water. These have been found useful in locating opal at Lightning Ridge and sought in other states. In addition, opal has been found associated with Cretaceous palaeochannels in Queensland and Lightning Ridge.

Variations in the types of opal depend on a number of factors. Firstly the climate provides alternating wet and dry periods, creating a rising or falling water table which concentrates silica in solution. Deep weathering of Cretaceous clay-rich sedimentary rock produces both silica and kaolin. Silica spheres are deposited in a regular array in voids from a receding water table forming precious opal in a variety of host materials. The orderly arrangement of the spheres in a regular three-dimensional array creates diffraction and interference, producing the brilliant colours of precious opal. The range of colours is controlled by the size of the silica spheres and by refraction at the surface of the opal.

Opal Classification

There are many different types of natural opals. There is black opal, white opal, crystal opal and jelly opal. There are other kinds - fire opal, hyalite, water opal, hydrophane and honey opal. But is there legitimately a semi-black or a grey opal? And if so, where does one start and the other end? Furthermore, there are doublets, triplets and treated or dyed matrix.

The problem facing us all - the buying public, the gem trade and the gemologist — is how to describe a gemstone that occurs both with and without a play-of-colour, in almost every colour of the rainbow, in every tone of lightness and darkness from black to white, and in every degree of transparency from opaque to perfectly transparent. Also this unique gemstone displays differences in mineralogy that reflect the varying geological environments in which it forms. So far there is no clear and unequivocal scheme of classification by which opal can be reasonably easily identified by any of the three groups of people mentioned.

Varieties of Natural Opals

The variety of natural opal is determined by the two characteristics of body tone and transparency.

Body Tone

Body tone, body of tone, background colour, colour background, body colour or body background all refer to the relative darkness or lightness of the opal, while ignoring its play of colour. This is assessed on a Scale of Body Tone (from N1 to N9).

Black Opal - is the family of opal which shows a play of colour within or on a black body tone when viewed face-up, and may be designated N1, N2, N3 or N4 on the Scale of Body Tone.

Dark Opal - is the family of opal which shows a play of colour within or on a dark body tone, when viewed face-up, and may be designated N5 or No using the Scale of Body Tone.

Light Opal - is the family of opal which shows a play of colour within or on a light body tone, when viewed face-up, and may be designated N7, N8, or N9 on the Scale of Body Tone. The N9 category is referred to as white opal.

Fire Opal - is opal with a rare distinctly coloured body (such as red, orange, yellow or brown). It is popular for its eye-catching colour. It can be classified as black, dark or light opal, by reference to the Scale of Body Tone, and also have a notation stating its distinctive hue appended to its determined body tone.

Transparency

Opal shows all forms of diaphaneity that range from transparent to translucent (semi-transparent) to opaque. Natural precious opal which is transparent to semi-transparent is known as crystal opal. Crystal opal can have either a black, dark or light body tone. In this context, the term 'crystal' refers to the appearance of the opal and not its crystalline structure.

Formation of Opal

Weathering Model – In the Lightning Ridge opal fields, most opal is recovered from near the top of the various Finch clay facies lenses (opal dirt), immediately below the overlying portion of the Wallangulla Sandstone Member. Geological mapping of the opal fields has shown that the sandstone is thickest in areas that have produced large amounts of opal. Opal is rarely produced from areas in which the sandstone is thin or absent.

This data supports a view that the weathered sandstone at Lightning Ridge is the source of silica for the formation of opal. The occurrence of opal in the uppermost portions of the Finch clay facies is consistent with the concept that siliceous solutions (groundwater + silica) percolated down from their source rock (Wallangulla Sandstone Member) and were trapped by the impervious barrier presented by the clay fades (opal dirt).

Interpretation of aerial photographs and satellite images shows numerous lineaments (representing faults or joints) throughout the Lightning Ridge area. The lineaments identified are commonly several kilometres in length and may be expressed at the surface by a number of large box or belah trees following the line of lineament. There is a good relationship between the occurrence of lineaments, particularly where they intersect, and the occurrence of opal deposits.

It is possible that these major lineaments were passageways that allowed deeper weathering and initial groundwater movement, and therefore controlled the sites of the development of the smaller-scale structural features, such as joints, faults, and subvertical breccia zones, which are observed in most of the workings in the Lightning Ridge area. Collectively, these features have created zones that allowed the passage of groundwater down from the surface, resulting in today's opal fields.

An additional factor necessary for the formation of opal is the appropriate chemical environment. This process is not well understood, but may involve:

• a change from an alkaline to an acidic environment;
• the presence of aluminum oxide, ferric oxide or magnesium oxide; and
• the presence of sodium chloride or sodium sulphate.

Fundamental concepts associated with the weathering model include:

• opal developed by weathering at some time after the rocks in which it is found;
• a source of silica, predominantly sandstone, is required for opal to form;
• a claystone permeability barrier trapped water carrying silica, which allowed the opal to form; and
• blows and faults generally enhanced the permeability of rock strata and increased the potential for opal to form in association with sandstone and claystone.

Syntectonic Model – This model proposes that opal formed from mineral-bearing waters rising towards the surface under pressure along fault and breccia pipes. Deformation of sandstone and claystone layers by large-scale, tectonic, geological processes caused water to be forced hydraulically into fractures and faults. As the water dissipated into areas of lower pressure, the opal was deposited as veins.

Some of the silica-rich water that formed opal also escaped to the surface as hot springs. Where this water infiltrated Tertiary sands and gravels, silcrete has been formed as a result. Accordingly, there may be an association between areas of silcrete and opal deposits.

Fundamental concepts associated with the syntectonic model include:

• opal was developed after the rocks in which it is found,
• opal was formed relatively quickly; and
• faults and blows are essential for opal to have formed in a given area.

Microbe Model – The opal-bearing claystone commonly contains substantial amounts of fine, fossilised organic matter. Various types of microbe fossils, primarily aerobic (air-dependent) bacteria, have been identified within samples of opal from the Lightning Ridge area. Although none of these fossil organisms are visible to the naked eye, microscopic studies have shown them to be quite abundant.

At the time the Cretaceous sediments were deposited, abundant organic matter and montmorillonite (smectite) clay within some sediments provided an ideal habitat for the microbes to feed and breed. Waste acids and enzymes excreted by the microbes caused the chemical weathering of clay minerals and feldspars in the surrounding rocks.

Ultimately, the ongoing feeding and waste production processes of the microbes created favorable physical and chemical conditions for the formation of opal.

Fundamental concepts associated with the microbe model include:

• Opal developed at the same time as the rocks in which it is found;
• microbes tended to be most abundant where there is an abundance of montmorillonite (smectite);
• and the biological activity of microbes created a chemical environment that promoted the weathering of clay minerals and feldspar.

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