Types of Opals

The Australian Opal and Gem Industry Association (AOGIA) has recently agreed on an official nomenclature for opal, which is being widely distributed and adopted by international bodies in the industry. for the full text of the nomenclature. Click below for a brief summary of the main types:

Natural Opal
Natural opals are those which have not been treated or added to in any way by mankind, other than by cutting and polishing. Natural opals are usually described as light, dark/black, boulder, and matrix. Although boulder opal has ironstone backing, it is regarded as a solid natural opal because this backing occurs naturally. The variety of natural opal is determined by the two characteristics of body tone and transparency.

Body Tone:
The base tones of light, dark and black opal range from colourless, white, through the various shades of grey, to black.
Transparency:
Opal of any body colour will be opaque, translucent or transparent. When it is transparent or very translucent, and the colour clarity is sharp, it is often referred to as crystal opal.

Light Opal
Natural opals with a base tone ranging from colourless to medium grey are called light opal. Some people refer to these as "white," although this expression should only be used where the body colour is very milky. Light opal makes up the bulk of precious opal. It comes from all opal fields, but today the majority is found in Coober Pedy, South Australia.

Black/Dark Opal
Black/dark opal shows a play of colour within or on a dark body tone, while the play of colour of a black opal is within or on a black body tone, when viewed from the face up. It can be crystal or opaque. Some black/dark opals have a light crystal colour bar on dark opal potch (colourless opal), giving the otherwise light opal a dark appearance. Even expensive black/dark opals may have only a very thin colour bar on black potch.
Most black/dark opal is found in the mines around Lightning Ridge, NSW. Because of its relative scarcity compared to light and even boulder opal, it tends to be more expensive, given equivalent colours, clarity and patterns. Black/dark opal exhibiting bright flashes of red is extremely rare.

Boulder Opal
Boulder is a variety of precious opal which has an ironstone host rock bonded naturally to the gem. Often just a thin vein of precious opal is present. It occurs in specific locations over a wide area of Queensland where the opal fills cracks or voids in ironstone boulders. Boulder opal can be black, dark or light depending on the appearance of the stone when viewing the presentation face.

Matrix Opal
The term matrix opal is commonly used where the opal is intimately diffused as infillings of pores or holes between grains of the host rock in which it was formed. Andamooka matrix opal is a porous material from Andamooka, South Australia, which is often treated to enhance the colour by depositing black carbon by chemical treatment in the pore spaces in the stone.

Composite Natural Opal (Doublets/Triplets)
Triplets and doublets are a combination of natural opals and artificial veneers.