Most boulder opals have an undulating face as the seams are usually not straight, which means the cutting of the boulder opal can be more tiresome and messy if you are looking for symmetrical shapes. Additionally, ironstone, when the cut is fairly dirty to cut on the wheel and combined with water, makes everything, including your surrounds turn brown!
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 opals have 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 two 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. Silica spheres fill up the cracks in the boulder are filled over time with the opal silica which we call opal.