"Boulder Opal" occurs spasmodically within a belt of Cretaceous sedimentary rocks from the New South Wales' border to about 1000 km north-west in Queensland, Australia, (the "Boulder belt"). It consists of opal on a dark ironstone (the host rock). The opal occurs as a thin uneven layer naturally adhering to the ironstone. Some ironstone may be visible in the surface when cut. However, a Boulder Opal which has a full face with no ironstone intrusions is said to be 'clean-faced' and commands a higher price.

These stones, often called Koroit and Yowah Matrix opals, depending on the area found, are boulder opal matrix that displays flashes of opal, lots of surface pinfire or just 'picture stones' that do not qualify as gem opals but are simply too good to waste!

It is very difficult to photograph as it is in turning the stone that the often sparkling 'pinfire' is seen. We've done our best, using a desk halogen lamp to give you an idea of each stone, but please bear in mind that many of them appear to have a lighter brown matrix than is actually the case.

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