Now the challenge is to cut it. In precious opal the arrangement of spheres is in orderly layers, and light passing through the spheres is diffracted at the void and layer interface to produce the vivid play of colour associated with opal. Larger silica pheres are associated with more sought after colours, such as red. It has hardness ranging between 5-6.5, is brittle with a conchoidal fracture and some light varieties fluoresce white or yellow under long and short wave ultraviolet light. This property is used on South Australian fields such as Coober Pedy to noodle opal from tailings, which are run on conveyor belts through darkened sheds, past UV lighting.
Black opal is generally more sought after and more valuable than light opal.
In Australia the major opal producing fields for black opal are Lightning Ridge in New South Wales and Mintabie in South Australia. The stones usually have light grey appearance when found due to a thin outer layer of grey potch. When stones are clipped they reveal black potch inside along with any colour bar.