The word opal means “precious stone”, and opals are primarily found in Australia. A distinction is made between opals with a white or black body color. The black opal is the most valuable, because it is rarer than the white. Color variations are due to light diffraction caused by the internal structure of the stone, which comprises spheres of cristobalite that are one ten-thousandth of a millimeter in diameter. It has a hardness of 5.5 - 6.5 on the Mohs scale.