Australian Boulder Opal
Opal is most famous for its play of color, a display of rainbow-like hues that change with the lighting or angle of observation. Not every kind of opal displays this unique property, only the opals that are known as precious opal. Lesser opals, such as common opal (also known as 'potch') and fire opal, have no play of color.
The most prized of the precious opals is the black opal of Australia, first discovered in the 1800s at Lightning Ridge in New South Wales. Though Australia produces nearly all the world's opal, 90% of the Australian opal produced is light colored opal.
The rare black opal accounts for only 8% of Australian opal. Black opal is especially valued because its dark body tone gives the play of color and extra vibrancy not seen in the more common white opals.
The remaining 2% of Australian opal is the second most prized form of opal, a unique form known as boulder opal. First discovered in Quilpie, in Western Queensland in about 1870, the boulder opal is found embedded in ironstone boulders. The opal usually forms as thin veins within these boulders, and most stones are cut to include some of the host ironstone matrix. Boulder opal is sometimes referred to as opal in matrix for this reason.
Boulder opal is especially attractive because, like the black opal, it has a dark body tone which adds vibrancy to the play of color. Boulder opal also has a higher density because of the ironstone content, and can be more durable as well.
Due to the thinness of the opal veins that form in the ironstone boulders, it is often impossible to cut domed cabochons in most boulder opal stones. The stones are therefore usually cut into free form, irregular shapes to maximize the size of the stone and minimize the loss of opal.
Boulder opal is still fairly new to the market, but has great potential. The area that has the potential to yield boulder opal is much larger than the black opal deposits, and a relatively small proportion of this has been explored. The area stretches along a 200 to 300 kilometer area in Western Queensland.