Types of Opal
Opal is known for its colour and is distinguished from other gems due to its ‘play of light’ or ‘fire’. This refers to the opal’s ability to show a vast array or all the colours the rainbow and are visible in the grains of the stone. When viewed, if the stone is tilted or rotated, the colours may change of disappear due to the change in direction of the light hitting the grain/s. This is what makes the opal so unique and strikingly beautiful.
Opals, those untreated other than than cutting and polishing are referred to as natural, while treated/modified Opals are refered to as 'man-made'.
Natural Opals are made up of:
- Natural – Light, Dark and Black
- Natural – Boulder
- Natural – Matrix
Man-Made Opals include:
- Man-Made - Treated Matrix
- Man-Made - Composites
- Man-Made - Synthetic
Opal classification is determined by ‘base/body tone’ and ‘transparency. Body tone refers to the stones tone which can range from colourless, white, and grey to black, while transparency refers to whether it is opaque, translucent or transparent. The clearer the transparency, the sharper the colour and these are often referred to as Crystal Opal.
Natural - Light Opals
Light Opals are those that exhibit colourless to medium grey in terms of base/body tone with many people classifying them as white (this should only be used if the specimen is in fact 'milky'). The Light Opal, which makes up the bulk of the precious opal are sourced primarily from Coober Pedy and Mintabie in the north of South Australia and to a less extent in White Cliffs (Outback NSW) where they were first discovered.
Natural - Black/Dark Opals
The most valued of all opals is the Black Opal, of which Lightning Ridge is ’home’ and their inherent value comes from their rarity. Black Opal is distinguished from other opals by their dark background (Body Tone) and this characteristic enables the brightness of colour of which they are known. The ‘darkness’ is the result of the opal is formed on a darker (black) quartz-like layer that enables greater refraction/reflection of the light to the top of the opal, especially the reds and pinks. It is the 'reds' that are more valuable.
To expand the 'play of color' of Black Opal even further, some specimens have a light crystal colour bar on dark opal potch (colourless opal) which gives the otherwise light opal a dark appearance. Even expensive black/dark opals may have only a very thin colour bar on black potch (Colourless Opal).
Black Opal is found as what is referred to as 'Nobbies', which are fossil replacements of corals or sponges. As the opal is formed, silica replaces the organic material and carbonaceous material or impurities like titanium impregnates the mineral structure giving the Black Opal its body colour.
Compared to Light and Boulder opals, Black Opals fetch a higher price for a given colour, clarity and pattern, due to their scarcity.
Natural - Boulder Opals
Boulder Opal is found sparsely distributed over a wide area of Queensland Australia, sandstone or ironstone boulder country where the Opal fills cracks and crevices in the ironstone boulders. Opal bearing boulder is always cut including the host ironstone. Boulder Opal is in very high demand and extremely precious.
Boulder Opal is usually cut to the contours of the Opal vein creating a baroque wavy surface and is often freeform and irregular in shape, making boulder Opal unique and exclusive among it's peers. In Boulder Opal there is a colour and pattern to suit everyone’s taste, from the brightest flashing red through to electric blues, greens, orange, pink, purple and many more in every shade you could imagine (every colour of the spectrum).
Boulder Opal, with colours ranging from black or light, is sourced from areas in Western Queensland, which are cut to incorporate the brown host Ironstone and are valuable due to their high demand. Around Andamooka in South Australia, where the host-rock is quartz which is lighter in colour, the Boulder Opal is also referred to as a ‘Painted Ladies’.
Natural - Matrix Opals
Matrix Opals are silicified sandstone or ironstone which has opal forming within infillings of pores or holes or between grains of the host rock.
Queensland Boulder Matrix Opal is distinguished by its ironstone host rock, while Andamooka Matrix Opal is a porous material from Andamooka, South Australia, which is often treated to enhance the colour by depositing black carbon via chemical treatment in the pore spaces in the stone. The Andamooka Matrix Opal is classified more as a made-made opal.
Man Made - Composites
Composites a slice of Light Opal and adhered to a back of black potch, plastic or boulder. While they are popular as souvenirs, they are not investment quality stones and lack the durability of the natural variety with the veneer of the opal and back vulnerable to splitting due to moisture and heat.
Composites can be triplets or doublets, with the former being a veneer of opal on a black backing, while the triplet is a doublet with protective layer glued to the top. Doublets, with generally more opal used in their construction, are generally more expensive than the triplet.