Although not referring to opals, the words used by 18th century English poet James Thomson summarise well the fascination many have with opals and their play-of-colour patterns:
“But who can paint
Like Nature? Can imagination boast,
Amid its gay creation, hues like hers?”
Each opal specimen is unique, and valuation is thus a challenging undertaking. In the following we will cover different quality factors for opal, based on criteria described in the book by Andrew and Damien Cody – The Opal Story- published in 2008.
- Type of opal: It is important to determine the type of opal. For instance, a dark boulder opal may produce a similar colour display to a black opal. However, the black opal will command a higher price.
- Brilliance: The degree of brightness is of paramount importance. An opal cannot be valuable without this attribute.
- Body tone: The degree of darkness in the body tone, in regard to black or boulder opal, is taken into consideration when assessing the value. A black body colour is more valuable than a grey or light body tone, when all other factors are equal.
- Transparency: White opal is more desirable when it has a degree of transparency (i.e. light shining through it) and if lively, brilliant colours are present in opals with a certain amount of transparency, they are most highly prized.
- Patterns: Interesting patterns of the diffracted colours are rare and, when combined with brilliance, can have an enormous impact on the value.
- Dominant diffracted colours: Great care needs to be taken when applying this factor, remembering that type and brilliance override all other factors – i.e.: a blue/green brilliant stone will usually be more valuable than a dull red stone. However, the dominant colour has decreasing value in this order: red, orange, yellow, green, blue, indigo and violet – with red and orange colours being the most valued. This rule is more particular to black opals. Strong red colour-play in a dark body-toned black opal is the most valuable.
- Thickness of the colour bar: The colour bar is the part of the opal that shows play-of-colour. Greater thickness of the bar enables a domed surface to be cut from the rough stone, thereby increasing value.
- Shapes: Precious opal is usually cut into oval, pear or round shapes. However, many opals are also cut in fancy shapes, thus preserving partly the shape of the rough opal specimen.
A selection of some of the incredible patterns that can be found in black opals. Source: Cody (2008).
An example of different sizes of colour bars (the area containing play-of-colour) in Australian black opal. Photos: Cody Opal.