Opal

 

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 opal as the stone is turned. There are also known rare patterns such as the harlequin (blocky pattern), Chinese writing and picture stones that display scenes and other images.
The term 'Play of Colour' is used to describe the movement and change of colour across the surface of an 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 below.
The brilliant flashes of coloured light that come from the surface of an opal are called 'fire' which may change in different types of light.
Sunlight is the best type of light in which to view an opal, followed by full spectrum non-diffused light.

Opal Source Locations

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
 
Black opal

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).
Although the 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 opal 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 & therefore valuable opal.

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.

 Black crystal opal
Dark crystal

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'.
The base colour present in dark opals is commonly a shade of grey or brown, Most Queensland boulder opal with a natural ironstone base is dark opal, although they are normally described simply as boulder opal (shown below).
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. 		White opal
Crystal opal

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 opal can be colourless or it may have a base colour. Crystal opals may be described using their 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 opal is turned or viewed from different angles. Bright multicoloured crystal opals are 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 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.
Although, only a small amount of precious opal is present to produce the colour, most jelly opals display opalescence (The lustre seen on the surface of a pearl) giving the opal a positive attribute.
The colour in jelly opal may not be distinct or clearly seen until the stone is rotated, sometimes needing different intensities of light.
As jelly opal is a colloquial term, materials with a wide range of colour intensities appear on the gem market, described as jelly opal.

 

 Jelly opal
Boulder opal

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.
When the opal is dispersed in lots of small cracks that run through the host material or appear as whirling lines or spots of colour. Then the opal is described as 'Boulder Matrix Opal' or 'Matrix Opal'. Boulder opal and matrix opal varieties 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 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.
Cabochons may be cut from all variations of the material, whereas transparent material 'without a play of colour' is normally used to produce facet cut stones. Mexican fire opal is bright material, well known in the gem market.

Fire opal
Opal shell fossil 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.

OPAL TYPES

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 Boulder
Opal.

Queensland 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

Opal Tone Chart

 

Properties of the Gemstone and Additional Information

Crystal system:

Amorphous (non-crystalline)

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 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 opals display 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 opals 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.

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.
Any enhancement applied to opal would have to be disclosed.

A light coloured matrix opal from Amdamooka in South Australia is treated with sugar and sulphuric acid, to darken the body tone and accentuate the play of colour. The result of the treatment gives the opal 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 opal 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 opal triplet is therefore quartz. Other materials, such as glass, may be used for the cap.