Opal is the national gemstone of Australia and deserves the title by virtue of its sheer beauty. This unique gemstone has many varieties and displays more colour pattern variations than any other gem material. The colour may range from one single colour in one opal to every colour imaginable in another. The colour may also vary from soft pastel colours to rich vivid colours, giving great variation to this beautiful gemstone.
Opal also displays special effects such as the ‘rolling flash’ where a flash of colour rolls across the surface of the gemstone as it is turned. There are also rarely found colour patterns such as the Harlequin (blocky pattern), Chinese writing and Picture Stones that display scenes and other images. Good examples of these opals fetch very high prices on the collectors market.
The term 'Play of Colour' is used to describe the movement and change of colour across the surface of the opal as it is turned; also from below the surface of transparent and translucent stones. The ‘play of colour’ does not come from impurities or pigments, it is caused by the diffraction and interference of white light through opals unique structure as described in Properties of the Gemstone.
The brilliant flashes of coloured light that come from the surface of high quality opal is called 'fire' and is best viewed in sunlight. On cloudy days a full spectrum non-diffused light (incandescent lamp) can be used to show similar results.
The majority of commercial opal comes from Australia (around 95%).
Coober Pedy in South Australia is known as a major source for high quality crystal and white-based opal. Black opals are also found but are rare for the area. Other mining areas in South Australian that produce high quality material are Mintabie, Andamooka and Lambina.
Queensland boulder opal is mined in Opalton, Winton, Quilpie, Eromanga, Jundah and other Queensland areas. Yawah and Koroit, also mined in Queensland, are well known for their beautiful 'nut opals', nut-shaped ironstone boulders that have a core of brilliant crystal opal.
Lightning Ridge in New South Wales is famous for it's black opal and high quality crystal opal.
Other source locations for opal include Brazil, Ethiopia, Guatemala, Honduras, Mexico (Mexican fire opal), Canada (British Columbia) and the USA (Nevada and Idaho).
Black opal is a natural opaque variety that shows a play of colour against a very dark to black background. To be a black opal, the body tone has to be in the range N1 to N4 on the Body Tone Scale (shown below).
Although black opal naturally occurs in one piece, it can be thought of as having a layer of highly transparent crystal opal on top of a layer of opaque black potch (potch = opal without a play of colour). The colours of the crystal layer appear highly saturated against the black background, with stunning effect. Black opal is the most precious and therefore valuable opal variety.
Black Crystal Opal is a transparent to semi-transparent variety that has a body tone also in the N1 to N4 range.
Black crystal opal often does not appear to be transparent, as the play of colour is so intense that it masks the transparency. Good specimens command a high price on the opal market, due to their rarity and inherent beauty.
Dark Opal has a body tone from N5 to N6 on the Body Tone Scale, bridging the gap between black and light material. The material may be opaque to transparent, when transparent to semi-transparent it would be termed a 'Dark Crystal Opal' or a 'Semi-Black Crystal Opal'.
The base colour present in dark material is commonly a shade of grey or brown, Most Queensland boulder opal with a natural ironstone base is a dark material, although they are normally described simply as boulder opal (shown below).
Light Opal has a body tone from N7 to N9 on the Body Tone Scale and when it occurs as N9 on the scale, the material is termed a white opal. When not white in colour the material is mainly grey or brown. Transparent to semi-transparent material is termed 'Light Crystal Opal'.
White material is opaque to translucent with a play of colour ranging from soft pastel colours to the bright vibrant colours described as 'gem grade'. Fossilised opal shells often produce exceptional high quality gemstones.
Some white opal will display sharp vibrant colours in direct light and soft pastel colours in diffused light.
Direct light - is sunlight and incandescent light (from filament bulbs and halogen lamps).
Diffused light - is the light seen on a cloudy day and the light from some fluorescent lamps.
Crystal Opal is a transparent to semi-transparent variety of precious opal. Although it is not a crystalline material, the term adequately describes the stones appearance with regard to its clarity. (in some specimens, crystal clear). The body tone of crystal material can be colourless or it may have a base colour. Crystal opal is described using it's base colour, for example, an opal with a yellow base colour would be classified as a 'Light Yellow Crystal Opal'.
A play of colour may be seen, not just on the surface of a crystal opal, but also from the interior of the stone. Various colour patterns may be displayed to the observer when the gemstone is turned or viewed from different angles. Bright multicoloured crystal material is magnificent.
The Opal Body Tone Chart determines whether a crystal opal is a 'Light' or 'Dark' variety.
Jelly Opal is a colloquial term used to describe highly transparent material that contains a small, to very small amount of pale colour. Some describe 'jelly opal' or 'jelly crystal opal' as having a small amount of subdued colour.
Although only a small amount of precious opal is present to produce the colour, most jelly material displays opalescence (The lustre seen on the surface of a pearl) giving the material a positive attribute.
The colour may not be distinct or clearly seen until the gemstone is rotated and often varying the light intensity is necessary to see the colour.
As jelly opal is a colloquial term, materials with a wide range of colour intensities appear on the gem market, all described as jelly opal.
Boulder Opal consists of a layer of opal naturally formed onto a host material. Queensland boulder opal is formed on ironstone rock (image left). The layer of precious opal may be very thin, with areas of polished ironstone often showing in the face of the finished gemstone. When a boulder opal has a layer of dark or black potch, between the ironstone and precious crystal opal layer, the colours appear brilliant. This gemstone is termed 'Black Boulder Opal'. and demands high prices.
When the material is dispersed in lots of small cracks that run through the host material or appear as whirling lines or spots of colour. Then the material is described as a 'Matrix Opal'. This variety may produce picture stones with diverse and often surprisingly, well organised natural colour patterns.
Fire Opal is a yellow to orange, to orange-red based material, that may or may not possess a play of colour. Therefore, the description 'fire' does not relate to a play of colour, but to the body tone and brightness of the material. It is commonly transparent to semi-transparent, but also occurs as translucent to opaque material. Colourless, white and reddish-brown material occurs and is also referred to as 'fire opal' in the gem market.
Transparent Fire opal without a play-of-colour is normally used to make facet cut stones. When a play of colour is present they are normally cut en cabochon. Mexican fire opal is well known in the gem market for it's bright highly saturated orangey-red colours.
Opalised Fossils including shells, clams, belemnites and other small sea creatures are commonly found in areas of Australia, where ancient sea beds once existed. Rarely, the opalised remains of larger prehistoric land and sea creatures are found. The bones and teeth of the plesiosaur, a large swimming reptile, around at the time of the dinosaurs, have been found in mines in different parts of Australia. When commonly found fossil material, such as sea shells, is found broken, it is normally cut as cabochons and carvings. Whereas complete fossil specimens are sought after by collectors and museums.
Potch is opal that does not display a play of colour and commonly occurs as white, grey and black material. Black potch is one of the materials used as a backing for doublet and triplet opals. Another commonly used material is obsidian (volcanic glass). Potch and potch with colour (where a small amount of precious opal is present) is a cheap material used by students to perfect their cutting and polishing skills.
This material is similar to potch but has a degree of crystallinity. It does not show a play of colour, but may show opalescence; the type of lustre seen on the surface of a pearl. Honey opal is a translucent common opal with a distinct honey-yellow colour. Common opal also occurs in other body colours including pink, blue and purple.
Pink opal from Peru has an attractive pink colour, it is popular in jewellery and for carvings. Rings feature Peruvian pink opal carved into flower shapes, a favourite is the pink rose.
Other Varieties of Opal
Varieties less seen in the jewellery market include:
Painted Ladies are large boulder opal specimens from Andamooka, South Australia. They are comprised of opal naturally formed on quartzite (tan coloured, metamorphosed sandstone), not ironstone as with Queensland boulder opal. The material looks as though it has been painted on the quartzite and colour patterns may resemble scenes, such as landscapes. They may also be painted to enhance the naturally formed design. Originally exclusive to Andamooka, painted ladies are now being sold from other mining areas in South Australia.
Hyrdrophane is a porous opal variety that, when dry, does not show a play of colour very well in some stones and not at all in others. In the dry state it can be opaque, but when soaked in water for over 10 minutes, good specimens will become transparent and may show brilliant colours. Becoming colourless again when it eventually dries out.
Hydrophane matrix opal from Andamooka, South Australia, is a light coloured porous material. It is dyed using a sugar and acid process to produce opals that appear as black opals. The process can produce very attractive gemstones.
Being porous allows this gem material to be treated with polymers to change the base colour and stabilise it's vulnerable structure, improving durability. Opal collectors store the material in water (in a glass container) so natural bright specimens can be displayed.
TERMS USED TO DESCRIBE OPAL
The terms used to describe opal vary throughout the industry and colloquial words are also in use. To standardise the way opal is classified and generally described, an 'Opal Nomenclature and Classification' document was produced by the following bodies:
Australian Gemstone Industry Council Inc.
Australian Gem Industry Association Ltd.
Gemmological Association of Australia Ltd.
Lightning Ridge Miners Association Ltd.
Jewellers Association of Australia Ltd.
The following is a basic description of opal with the above document in mind.
Natural Opal Type 1 Type 1 is comprised of a single piece of material, that may be cut and polished, but not enhanced in any way.
Natural Opal Type 2 Type 2 is comprised of a single piece of material, where opal is naturally formed on a host rock.
Queensland boulder opal and Andamooka Painted Ladies (explained above) are natural opal type 2.
Natural Opal Type 3Type 3 is comprised of a single piece of material in which small pieces of opal are dispersed throughout a host rock or in very small streaks running through the rock. Commonly known as Matrix Opal.
Matrix opal is found in Queensland and Andamooka in Australia and other parts of the world including Mexico, Brazil and Honduras.
Opal Variety Classification
The opal varieties described above are determined by their body tone and transparency. The body tone is categorised in the AGIA (Australian Gemstone Industry Association Ltd) Opal Body Tone Scale, shown below.
To see the body tone of an opal you have to imagine the opal with the play of colour (fire) removed and visualise the background colour and tone only. In the case of some crystal opal, there will be no colour but a tone only, seen as a shade of light to dark.
The scale below is for appreciation only as the shades will vary between different video screens.
OPAL BODY TONE CHART
Properties of the Gemstone and Additional Information
Although amorphous, opal has a unique structure. It is this structure that causes the 'Play of Colour' in precious opal.
Opal is comprised of sub-microscopic silica spheres bonded by water. When the spheres occur in a random non-aligned arrangement, only a base colour is seen; this material is called potch.
When the silica spheres are aligned in an orderly three dimensional arrangement or stack, then the spheres and water-filled gaps between the spheres, act as a diffraction grating. White incident light entering this structure is diffracted, splitting the white light into component wavelengths to produce individual colours; this material is called Precious opal.
The actual colour observed is determined by the size of the silica spheres and the water filled gaps (as the spheres increase in size, then so do the water-filled gaps).
The 'play of colour' effect, is the appearance of the colour changing and moving across the stone as it is turned. This is caused by the incident light entering the opal structure at different angles, displaying different parts of the spectrum to the observer. Even single coloured stones show a moving effect, as the colour lessens or disappears against the background in one place and appears in another.
There is also a light interference effect from the orderly layers of silica spheres, that produce the brilliant colours in opal. Light is refracted from lower layers of the opal stack, to meet up and synchronise with light refracted at higher layers. When colours of the same wavelength are transmitted in synchronisation, they are reinforced producing brilliant colours. The most expensive material displays magnificent vivid colours.
Therefore, the colour and brilliance of precious opal does not come from mineral colouring agents, but is caused by the diffraction and interference of white light from within the materials unique structure.
It is intriguing to consider the formation of opals with special effects such as the 'Rolling flash' or the rare ‘Harlequin’ pattern with almost perfect blocks of alternating colour (in good examples). How such effects and patterns could evolve naturally, adds to the mystique of opal.
Hydrated silica dioxide, SiO2.nH20. Typically contains 6% to 10% water.
Base colours are white, grey, brown, black and colourless. Fire opal occurs in all colours.
5½ to 6 on the Mohs' scale of hardness.
Poor: The material is prone to thermal shock (sudden temperature change). Rings should be worn with care to prevent them being physically knocked or stressed.
1.98 to 2.25
1.44 to 1.46
Most natural material is not treated, as there is no need to do so. However, some may have been treated to fill cracks or hide surface crazing, by oiling, waxing or polymer type treatments. Any enhancement applied would have to be disclosed.
A light coloured matrix opal from Andamooka in South Australia is treated using a sugar and sulphuric acid process. This darkens the body tone and accentuates the play of colour, giving the material the appearance of a beautiful black opal with intensely bright fire.
Following treatment, the opal is called Andamooka Treated Matrix Opal.
Opal doublets are made by gluing a slice of opal to black potch, obsidian or other suitable backing material. The faced-up surface of the doublet is therefore natural opal.
Opal triplets are conventionally made by sandwiching a thin slice of precious opal between a dark material (for the base layer) and a quartz cap (for the top layer). The faced-up surface of the triplet is therefore quartz. Other materials, such as glass, may be used for the cap.