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. 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 opal and white-based opal. Black opals are also found but are rare for the area. Other mining areas in South Australian that produce high quailty opals are Mintabie, Andamooka and Lambina. Queensland boulder opal is mined in Opalton, Winton, Quilpie, Eromanga, Jundah and other Queensland areas. Yawah and Koroit, also in Queensland, are well known for their beautiful 'nut opals', nut-shaped ironestone 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 countries around the world that produce opal include Brazil, Ethiopia, Guatemala, Honduras, Mexico (Mexican fire opal), Canada (British Columbia) and the USA (Nevada and Idaho). |
OPAL VARIETIES |
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Black opal is a natural opaque variety of opal that shows a play of colour against a very dark to black background. To be classified as a black opal, the body tone has to be in the range N1 to N4 on the Opal Body Tone Scale (shown below). |
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Black Crystal Opal is a transparent to semi-transparent variety of opal that has a body tone also in the N1 to N4 range. Often a black crystal opal does not appear to be transparent, as the play of colour is so intense that it masks the transparency. Good specimens of these opals are also of high value, due to their rarity and inherent beauty. |
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Dark Opal has a body tone from N5 to N6 on the Opal Body Tone Scale, bridging the gap between black and light opal. The opal may be opaque to transparent, when transparent to semi-transparent the opal would be termed a 'Dark Crystal Opal' or a 'Semi-Black Crystal Opal'. |
| Light Opal has a body tone from N7 to N9 on the Opal Body Tone Scale and when it occurs as N9 on the scale, the opal is termed a white opal. The opal may be opaque to transparent The base colour, when not white, is mainly grey or brown. Transparent to semi-transparent material is termed 'Light Crystal Opal'. White opal is opaque to translucent with a play of colour ranging from soft pastel colours to the bright vibrant colours seen in gem grade material. Fossilised opal shells often produce exceptional high quality white opal. |
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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 |
Jelly Opal is a colloquial term used to describe highly transparent opal that contains a small, to very small amount of pale colour. Some describe 'jelly opal' as having no colouir and 'jelly crystal opal' as having a small amount of subdued colour.
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Boulder Opal consists of a layer of opal naturally formed onto a host material.
Queensland boulder opal is opal formed on ironstone rock (image left). The layer of opal may be very thin, with areas of polished ironstone often showing in the face of the finished opal. 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 opal is 'Black Boulder Opal'. and demands high prices. |
Fire Opal is a yellow to orange, to orange-red based opal, 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 opal. It is commonly transparent to semi-transparent, but also occurs as translucent to opaque material. Colourless, white and reddish-brown material is also found and referred to as fire opal on the gem market. |
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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, has been in opal mines in different parts of Australia. When commonly found fossil material is broken, it is normally cut as cabochons and carvings. Whereas complete fossil specimens are sought after by collectors and museums. |
Potch
Potch is opal that does not display a play of colour and is 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 opal cutting students to perfect their skills.
Common opal
Common opal 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 peal.
Honey opal is a common opal that is translucent and has a distinct honey-yellow colour. Other
body colours also occur in common opal.
Other Varieties of Opal |
Opal varieties less seen in the jewellery market include:
Painted Ladies
Large boulder opal specimens from Andamooka, South Australia. Painted ladies are opal naturally formed on quartzite
(tan coloured,
metamorphosed sandstone), not ironstone as with Queensland boulder opal. The opal 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 being sold from other opal mining areas in South
Australia.
Hydrophane opal
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, some specimens will become as clear as crystal opal and may show brilliant
colours. Becoming colourless again when it opal dries out.
Hydrophane matrix opal from Andamooka, South Australia, is a light coloured porous opal. It is dyed to produce opals that appear as black
opals. The result can produce very attractive gemstones.
Apart from dying, hydrophane may be treated with polymers to change the base colour and stabilise or be left in water (in a glass
container) as bright opal specimens.
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
This type of opal is presented as a single piece; it may be cut and polished, but not enhanced in any way.Natural Opal Type 2
This type of opal is presented as a single piece and is comprised of opal naturally formed on a host rock. This type is known as BoulderQueensland boulder opal and Anamooka Painted Ladies (explained above) are natural opal type 2.
Natural Opal Type 3
This type of opal is presented as one piece of material in which small pieces of opal are dispersed throughout a host rock or in very
small streaks running through the rock. This type of opal is 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 backgound
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 is for appreciation only as the shades will vary between different video screens.
OPAL BODY TONE CHART |

Properties of the Gemstone and Additional Information |
Crystal system: |
Amorphous (non-crystalline) 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. |
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Chemical composition: |
Hydrated silica dioxide, SiO2.nH20. Precious opal typically contains 6 to 10% water. |
Colour: |
Opal base colours are white, grey, brown, black and colourless. Fire opal occurs in all colours. |
Lustre: |
Vitreous |
Pleochroism: |
None |
Dispersion: |
None |
Hardness: |
5 1/2 to 6 on the Mohs' scale of hardness.. |
Toughness: |
Poor: Opals are prone to thermal shock (sudden temperature change). Opal rings should be worn with care to prevent being physically knocked or stressed. |
Cleavage: |
None |
Density in gm/cc: |
1.98 to 2.25 |
Double refraction: |
None |
Refractive index: |
1.44 to 1.46 |
Common treatments: |
Most natural opal is not treated, as there is no need to do so. However, some opal may have been treated to fill cracks or hide surface crazing, by oiling, waxing or polymer type treatments. 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 opal triplet is therefore quartz. Other materials, such as glass, may be used for the cap. |