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 interference and diffraction of white light through opals unique structure, as described below.
The majority of commercial opal comes from Australia (around 95%) other areas of supply are Brazil, Ethiopia, Guatemala, Honduras, Mexico (Mexican fire opal) and the USA (Nevada and Idaho).
| Gemstone type: |
The terms used to describe opal vary throughout the industry and colloquial words are also in use. Australian Gemstone Industry Council Inc. 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. 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. 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.
Black opal Dark Opal Light Opal 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). |
| Crystal System: |
Amorphous (non-crystalline) |
| Chemical Composition: | Hydrated silica dioxide, SiO2.nH20 |
| 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. |
| 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: | 2.65 to 3.00 |
| Double refraction: | None |
| Refractive index: | 1.44 to 1.46 |
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 may be 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.