It is important to know the type of opal. For instance, a black boulder opal could in all respects be the same as a black opal, except that the black opal will be approximately three times as valuable.

The degree of brightness is of paramount importance. An opal cannot be valuable without this attribute. A gem that glows but is average in all other respects will still command a high price whereas a dull stone with a very good pattern such as a harlequin will only bring an average price. The photographs used in the grading scales of this book depict the brilliance and intensity gradient.

Good patterns of the diffracted colours when combined with brilliance have an enormous impact on the value. Pinfire and small type patterns are less desirable than broad patterns or large flashes. Distinct patterns such as rolling flash, straw pattern, Chinese writing, ribbon and harlequin are very rare and considered collectors' items.

With black opal or black boulder opal the degree of darkness in the body colour must be taken into consideration. The blacker the body colour the more valuable the gem.

Light opal is more desirable when it has a degree of transparency and if lively brilliant colours are present in crystal opals, they are most highly prized. However, transparency in black opal will generally reduce its value.

Great care needs to be taken when applying this factor, remembering that brilliance overrides all other factors. A blue/green brilliant stone will usually be more valuable than a dull red stone. The dominant colour has value in this order: red, orange, yellow green, blue, indigo and violet. The red orange colours are the more valued. This rule is more particular to black opal, 'red on black' being the most valuable.

Black opals and black boulder opals with a very thin colour bar are frequently worth many thousands of dollars a carat, although they would be worth more if they had exactly the same appearance with a thicker colour bar. Thickness of bar would also enable a cabochon to be cut from the rough, thereby increasing value.

Oval, pear and other regular shaped stones will bring a higher price than irregular shapes (freeshapes). Although it is possible to obtain greater yields from freeshapes, the demand is higher for the regular shapes. A cabochon enhances the appearance and signifies a thicker colour bar. A domed stone is therefore more valuable than a flat stone. Black opals do not usually have a cabochon.

Smaller sizes under a carat are worth less per carat than larger stones. This is particularly true with black opal, although less important if the stones are calibrated. Larger stones are harder to obtain and can sometimes command a premium.'