Ruby Mill Limited - Gemstone Guide

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 attached to a host rock. This type is commonly known as Boulder Opal.

Queensland boulder opal is a well-known natural opal type 2. The opal layer (often very thin) is naturally formed on an ironstone rock or in seams running throughout the ironstone. The colours can be brilliant especially when there is a dark to black potch layer between the ironstone and opal layer. When the body tone ranges from N1 to N4 on the Body Tone Scale (explained below), it is termed a Black Boulder Opal.
Painted Lady is a boulder opal from Andamooka in South Australia. The opal is formed on quartzite, not ironstone. This boulder type normally occurs in large pieces and are sold as 'picture stones'. They may be painted to enhance the naturally formed design.

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 VARIETIES

Opal varieties are determined by their body tone and transparency. The body tone is categorised in the AGIA (Australian Gemstone Industry Association Ltd) Body Tone Scale.

The scale shown below is for appreciation only as the shades will vary between different video screens.

Black opal
Black opal is an natural opaque stone that shows a play of colour against a very dark to black background. The body tone of black opal may be from N1 to N4.
Although the stone naturally occurs in one piece, it can be thought of as having a layer of precious crystal opal on top of a dark 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. This is the most precious and valuable opal.
Black crystal opal is transparent opal that has a body tone also in the N1 to N4 range.
Often a black crystal opal does not appear transparent, as the play of colour is so intense that it masks the transparency.

Dark Opal
Dark opal has a body tone from N5 to N6, bridging the gap between black and light opal.

Light Opal
Light opal has a body tone from N7 to N9 and when it occurs as N9 on the scale the opal is termed a white 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. Fossilized shells often produce exceptional high quality white opal.

Crystal Opal
When a precious opal variety is transparent to semi-transparent it is termed Crystal Opal. Although it is not a crystalline material, the term adequately describes the stones appearance with regard to its clarity. A play of colour may be seen, not just from the surface of a crystal opal, but also from the interior of the stone. Various patterns may be displayed to the observer when the opal is turned. Multicoloured crystal opals can be magnificent.
A crystal opal with a yellow body tone, would be classified as a Light Yellow Crystal Opal.

Fire opal is an orange to red-based opal that may or may not possess a play of colour. Therefore, the description 'fire' does not relate to a bright play of colour, but to the body tone colour of the opal. It is a transparent material and is normally faceted.

Jelly Opal is a colloquial term used to describe highly transparent opal with a play of colour in small or very small amounts. Some only display opalescence (the type of lustre seen on the surface of a peal).

Opalised fossils such as shells and belemnites are commonly found in areas of Australia, where ancient seabeds once existed. Rarely, the opalised remains of prehistoric animals and larger sea creatures are found.

Potch is opal that does not display a play of colour. Black potch is one of the materials used as a backing for doublet opals.

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.

Amorphous (non-crystalline)

Although amorphous, opal has a unique structure that enables a 'Play of Colour'.

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 the water-filled gaps act as a diffraction grating. White incident light entering this structure is diffracted, splitting the white light to produce colour; this material is called Precious opal.
The actual colour observed is determined by the size of the silica spheres, 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, this effect reinforces colour to produce brilliance. The resulting colours can be magnificent.

Therefore, the colour of precious opal does not come from mineral colouring agents, but from the diffraction of white light from it's 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 it's almost perfect blocks of alternating colour (in good examples). How such effects and patterns could evolve naturally, adds to the mystique of opal.