Opal is one of the worlds most rare and unique gemstones. The combinations of colours and patterns within each stone makes opal a desirable gemstone for unique jewellery designs with no two stones being alike. As the colours are generated by light diffusion, opal is a stone that wants to be ‘moving’. It is well suited to being worn as jewellery as movement causes the colours to flash and change, catching the eye.
Opal is Australia’s national gemstone, with 80-90% of the world’s production of precious opal originating here. The extraction of precious opal is still largely an amateur mining pursuit, with the majority of miners being individuals or small partnerships working on small-scale claims.
Opal is thought to be formed by water seeping down through layers of sandstone, where it absorbs soluble silica. As the water comes to rest in pockets and crevices underground, the water slowly evaporates, leaving behind the hardened silica. The silica is arranged in layers of microscopic spheres, with the size of the spheres and their orientation providing the colours and pattern arrangements seen in precious opal. Small spheres produce purples and blues, with the rarer large spheres showing reds. A combination of large spheres and small spheres in the same stone is very rare, with these red and blue combination stones being highly prized.
Opal types
Black opal
The rarest and most highly prized form of opal, the dark body-tone contrast beautifully with the showy colours. Mostly mined from Lightning Ridge, with somewhere around 90% of world production coming from this location
Crystal opal
Increasing in popularity due to its tendency for having super bright and flashy colour. Found in most opal producing areas, commonly Lightning Ridge, Coober Pedy and Mintabie.
White Opal
The most common opal type, the majority of quality white material is found in Coober Pedy, with White Cliffs, Lightning Ridge and Mintabie all producing material on a regular basis.
Boulder Opal
A unique form of opal, often found in ironstone host rock found in the south and central west of Queensland. Usually very bright in colour and makes attractive jewellery, it can be any body-tone, from white though to black. Top examples are valuable.
Matrix opal
Opal formed in a host rock. Often seen as flashes of colour embedded in a porous host. Commonly associated with Andamooka. Is often treated to make the host sandstone darker to bring out the colour play. Not considered gem-grade material.
Opal Grades
Apart from Colour, Pattern and size, opal can be classified by the attributes of Body Tone and Brightness.
Body Tone
Body tone is graded as the background shade of the stone when viewed from the face. An N scale is used to identify body tone, with N1 being black and N9 being white according to the guide below.
Brightness
Brightness is the strength of the colours in the face of the stone, with a B scale being used to classify the brightness according to the guide below
