Natural Opal - is opal which was formed in natural environment and has not been treated or enhanced in any way other than by cutting and polishing.
Synthetic Opal - is material which has essentially the same chemical composition and physical structure as natural opal but has been made by a laboratory or industrial process. Synthetic opal composites exist as synthetic opal doublets, triplets or mosaics. These must be disclosed as synthetic opal composites.
Imitation Opal - is material which imitates the play of colour of natural opal, but does not have the same physical and chemical structure or gemmological constants as natural opal.
Types of Natural Opals
There are many different types of natural opals. There is black opal, white opal, crystal opal and jelly opal. There are other kinds - fire opal, hyalite, water opal, hydrophane and honey opal. But is there legitimately a semi-black or a grey opal? And if so, where does one start and the other end? Furthermore, there are doublets, triplets and treated or dyed matrix.
The problem facing us all - the buying public, the gem trade and the gemmologist — is how to describe a gemstone that occurs both with and without a play-of-colour, in almost every colour of the rainbow, in every tone of lightness and darkness from black to white, and in every degree of transparency from opaque to perfectly transparent. Also this unique gemstone displays differences in mineralogy that reflect the varying geological environments in which it forms. So far there is no clear and unequivocal scheme of classification by which opal can be reasonably easily identified by any of the three groups of people mentioned.
The following classification is mainly based on Anthony Smallwood (1997), Chairman of then G.A.A. Opal Nomenclature Sub-committee.1. Based on chemical composition and homogeneity
- All Opal (Natural Opal Type 1) - is opal presented in one piece in its natural state apart from cutting or polishing, and is of substantially homogenous chemical composition.
- Boulder Opal (Natural Opal Type 2) - is opal presented in one piece where the opal is naturally attached to the host rock in which it was formed and the host rock is of a different chemical composition. This opal is commonly known as boulder opal.
- Matrix Opal (Natural Opal Type 3) - is opal presented in one piece where the opal is intimately diffused as infillings of pores or holes or between grains of the host rock in which it was formed. This opal is commonly known as matrix opal.
- Precious Opal - is opal which exhibits the phenomenon known as play of colour, which is produced by the diffraction of white light through a micro-structure of orderly arrayed spheres of silica.
- Common Opal and Potch - is opal which does not exhibit a play of colour. The distinction between common opal and potch is based on their formation and structure. Potch is structurally similar to precious opal but has a disorderly arrangement of silica spheres. Common opal shows some degree of micro-crystallinity.
- Hyalite or Mullers Glass, is a colourless opal which gives the appearance of glass. Rarely, it does display a faint tint of colour (blue, green or yellow).
- Hydrophane is an opaque porous opal which becomes transparent when immersed in water.
- Resin Opal is black or brown with a resinous lustre.
- Potch is a term used to describe common opal found on the opal fields. It is generally opaque and can be milky white, pale to dark grey, bluish grey, or black. "Magpie" potch is made up of black and white patches. A clear amber variety of potch has been found at Lightning Ridge.
3. Based on treatment methods
This is much more of an arbitrary rather than a natural classification. It is often used for commercial purposes.
- Untreated Opals/Free-form Opals - No treatment has been applied at all. Untreated opals are in their natural forms without sophisticated cutting or polishing. They are either free-form or free-form undulating in shape.
During the late 1990s, it was recognised by some opal miners and cutters that the recognised practise of cutting opal into perfect oval shapes was wasteful of some very good quality opal. As some of the best quality opal was then becoming scarcer, the concept of promoting free-form opal was initiated. The Opal Jewellery Design Awards commenced and promoted the use of opal cut into free-form with an undulating surface. This style of cutting lent itself to quite imaginative jewellery design opportunities and became popular with a worldwide market. The preparation of the finished opal from the rough state required more time and expense as the opal is usually carved and polished using a dremmel.
- Treated Opals are often sophistically cut, polished and/or cemented or attached to other materials. They include solids and composite opals. Composite natural opals consist of natural opal laminates, manually cemented or attached to another material. The opal component is natural opal. Composite opals include doublets, triplets, mosaics and chips.
- Solid Opals - must be cut and/or polished from the natural stone without being treated or added to in any other way. Soild opals are the most valuable type of opals. They are often round, long oval, oval or short oval in shape. However, solid opals in their natural forms without sophisticated cutting or polishing can be referred to free form opals. They are either freeform or freeform undulating in shape.
- Doublet Opals - are made by gluing slices of precious light opal to a common opal or other backing with blackened cement, usually an epoxy resin. The dark backing enhances the colours of the opal.
- Triplet Opals - are made by adding a clear protective cap of quartz or crystal glass to a doublet. The slices of precious opal used for triplets are thinner than for doublets and so triplets are usually less expensive. Doublet or triplet opals being layered, should not be brought into contact with detergent or oils as the glues could be affected.
- Mosaic and Chip Opals - are a composition of small flat or irregularly shaped pieces of natural opal cemented as a mosaic tile on a dark base material or encompassed in a resin.
Varieties of Natural Opals
The variety of natural opal is determined by the two characteristics of body tone and transparency.
Body tone, body of tone, background colour, colour background, body colour or body background all refer to the relative darkness or lightness of the opal, while ignoring its play of colour. This is assessed on a Scale of Body Tone (from N1 to N9).
- Black Opal - is the family of opal which shows a play of colour within or on a black body tone when viewed face-up, and may be designated N1, N2, N3 or N4 on the Scale of Body Tone.
- Dark Opal - is the family of opal which shows a play of colour within or on a dark body tone, when viewed face-up, and may be designated N5 or No using the Scale of Body Tone.
- Light Opal - is the family of opal which shows a play of colour within or on a light body tone, when viewed face-up, and may be designated N7, N8, or N9 on the Scale of Body Tone. The N9 category is referred to as white opal.
Fire Opal is opal with a rare distinctly coloured body (such as red, orange, yellow or brown). It is popular for its eye-catching colour. It can be classified as black, dark or light opal, by reference to the Scale of Body Tone, and also have a notation stating its distinctive hue appended to its determined body tone.
Opal shows all forms of diaphaneity that range from transparent to translucent (semi-transparent) to opaque. Natural precious opal which is transparent to semi-transparent is known as crystal opal. Crystal opal can have either a black, dark or light body tone. In this context, the term 'crystal' refers to the appearance of the opal and not its crystalline structure.
Black Opal (Fire Opal)
Dark Opals (Semi-black Opals)
Free Form Opal