What is boulder Opal?

Boulder Opal is opal which forms inside an ironstone concretion. The concretion was formed due to ionisation, from sedimentary deposition. By definition, they are ionised concretions of varying hardness with an approximate opal composition of SiO2at 28%, Fe2O3 + AL203 at 68% and H2O at 1% composition.

The opal forms within the cavities of the concretion in both vertical and horizontal cracks. Boulders vary in shape and size, from elongated to ellipsoidal in shape and from as small as a pea, too as big as a family car.

Boulder Opal has a tendency to cleave; when cleaved the "split" as it often referred leaves the beholder with two faces of opal, with a naturally polished face. This phenomenon is due to the angle the microstructure of the opal originally grew.

The opal cutter commonly finishes the opal, with the natural opal veneer exposed with ironstone backing. Although Queensland produces other forms of opal, sandstone opal as pipe and seam and matrix, which are often found at the same mine, it is the Boulder Opal that is most sought.

Opal is still being studied to try and understand its genesis, it has hard to believe that after a century of mining in Australia that the genesis of opal is still not clearly understood.