This is the most common type of opal. It is found mainly in Coober Pedy, South Australia. Coober Pedy is known as the Opal Capital of the world, it is the world's largest opal field. There are two types of white opal, milky and crystal. Milky opal is opaque, with the colours visible on the surface only, where as crystal opal is transparent, the colors being visible from within the depths of the stone.