Boulder Opal comes from multiple places in Australia. At one point much of Australia was a sea. It eventually dried up and formed rock and silica rich water infiltrated the stone and formed opal in the pockets.
The cabochon measures 26 x 18 x 7.4 mm. It has a girdle height approx. 3.1 mm and is sloped at 5 degrees for ease in setting. The back is also polished.
