The value of an opal is determined by the type of opal, the predominant colours it exhibits, the clarity or brilliance of these colours and the patterns in which the colours are arrayed. Good patterns of the diffracted colours have an enormous impact on the value of the opal.

Pinfire and small type patterns are more common, and thus less expensive than broad patterns or large flashes. Distinct patterns such as rolling flash, straw pattern, Chinese writing, ribbon, and especially harlequin,are very rare and considered collectors' items.

From Australian Precious Opal--A Guidebook for Professionals by Andrew Cody