Light Opal:
White or light appearance. Most abundant and affordable

Crystal Opal:
Transluscent appearance - a printed page would be roughly visible through the stone. Relatively abundant.

Semi-Black Opal:
Precious opal with a grey or darkish appearance when viewed from the front.

Black Opal:
Precious opal that has a black appearance when viewed from the front. Only opal that looks as dark as the N1-N4 body tone scale (below) can be truly called a black opal.Rarer than light or dark opal