Boulder opal has become a more popular type of opal next to black opal from Lightning Ridge. And can reach prices almost as high as black opal.
Most boulder opal has undulating faces as the seams are usually not straight. So the cutting of the boulder opal can be more tiresome and messy. The ironstone when being cut on the wheel is a dirty type of opal to cut. When you slice boulder opal, the brown spray goes all over the room and yourself.
Boulder opal can be light or dark like black opal and has many variations of body tone. Some boulder opal can have ironstone in the face, which devalues the opal quite significantly. Most boulder opal has to be split in half by using a flat head screwdriver and a saw. Along every vein, there is usually a crack between the color. If you make a small cut into the edge of the vein of color (2mm) then you can place the screwdriver into the cut and twist, you will usually get what we call a split with 2 pieces with matching opal pattern. This is the easiest way to face boulder opal. If the vein has no crack through it, you must grind one side off the ironstone till you get to the color bar. Smaller polishing equipment is needed to get into the curves that boulder opal will produce.
Australian Boulder is found in many different mining fields in Queensland and some of the most common places are Aramanga, Coroit, Quilpy and Yowah. There is much more exploration needed to find more boulder opal deposits. And since the fields are very spread out, the possibility for large deposits are looking good. Boulder opal comes from only one area in the world and that is Queensland in Australia.
The opal is formed in a host rock called boulder or ironstone. The cracks in the boulder are filled up by silica spheres are filled over time with the opal silica which we call opal.