Common opal is classified as non-gem quality opal. There are several varieties of common opal; most are opaque and none exhibit any 'play of colour'.
Hyalite, or Mullers Glass, is a colourless opal that gives the appearance of glass. Rarely, it does display a faint tint of colour (blue, green or yellow)
Hydrophane is an opaque porous opal that becomes transparent when immersed in water.
Resin opal is black or brown with a resinous lustre.
Potch is generally opaque and can be milky white, pale to dark grey, bluish grey or black. Magpie potch is made up of black and white patches. A clear amber variety of potch has been found at Lightning Ridge.