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 an 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.
Black OpalsBlack Opal is the most valued of Opals and comes mainly from Lightning Ridge. High quality stones are very rare. Easily distinguished by the blackness of the background "body tone" or body colour.
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.
Light OpalsNatural 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. White Opal may be transparent through to nearly opaque, although it usually has a "milky" appearance and has a light body tone or white body colour.
Mainly mined at Coober Pedy and Mintabie, (South Australia) although the first deposits were found at White Cliffs (NSW).
Boulder OpalsBoulder is a variety of precious opal that has the host rock forming naturally as part of the gem. Often just a thin vein of precious opal is present. It mainly occurs in specific locations over a wide area of Western Queensland. Boulder opal occurs as in-fillings of cracks or voids usually in ironstone boulders. Boulder opal can be black or light depending on the appearance of the stone when viewed from the surface.
The popularity of this type has soared since the mid 1970s. Boulder opals are invariably cut incorporating the host brown ironstone; they are in high demand and can be extremely valuable. A variety of boulder opal also occurs in Andamooka , South Australia , where the host rock is quartzite. These are called "painted ladies", but are normally only suitable as specimens.
Matrix OpalsThe 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. Boulder matrix opal is found in Queensland and can be distinguished by the ironstone host rock.
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.
