Black opal has a black tone underneath the colour. The blacker the tone the more intense the colour can be. Black opal is type 1 (see Terminology).
Dark opal has a dark grey - to light grey tone underneath the colour. The blacker the tone the more intense the colour can be. Dark opal is type 1 (see Terminology).
Crystal opal has a high degree of transparency throughout the stone. The play of colour can vary from slight to intense. Crystal opal is type 1 (see Terminology).
Light opal has a light tone (close to white but not necessarily white). A small degree of transparency. Light opal is type 1 (see Terminology).
Boulder Opal is only found in Queensland. When the ironstone boulders form they also have empty cracks. The water/silica solution finds its way into the cracks and then then in time the opal forms. Because the cracks are thin we do not try and remove the opal from the host roc...
Matrix Opal is a variant of boulder opal except rather than having the opal on the surface of the stone. The stone surface is a mixture of opal and ironstone (host rock). The patten can vary from a thousands of tiny sparkling flecks to swirling majestic patterns.
