It is the interaction of the precious opal with light that creates the sparks and flashes of the blues, greens, yellows, oranges, reds and purples in the various distributions of colours. They can be squares, lines, harlequins, and picture stones. Not every opal will display all of the rainbow colours.

The most common colour is blue, followed by green, and the rarest, and therefore the more expensive and desirable, is the red. The background of the opal also varies, from a milky white, to clear, to brown, to grey and to black. These are known as white opal, water opal, common opal, potch, and ironstone opal. Black opal commands the highest prices as the black background really favours the differing play of colour in the sunlight.