The Valuation of an opal is far from an exact science – being a very intricate and diverse form of precious gemstone, the criteria upon which an opal’s value is based are quite broad; and to complicate things there are often subjective elements involved in the assessment of the opal’s presented pattern, body tone and colour play that over the years many organisations have made great strides to standardise for the purposes of achieving an objective accuracy and fairness within the industry.
So Where to Start?
It’s important to know before we get into specific detail that the king of metrics for an opal’s value is a composite trait known as the opal’s play-of-colour.
Opals without this play-of-colour are called common opals or potch. This often is the same chemical material as precious Opal gemstones, but with a different internal structure, this results in a stone of a single solid colour.
Precious opals by comparison display a variety of colours that change constantly based on the viewing direction and availability of light. This is caused by the spheres of silicon within the opal being arranged in such a way as to diffract and diffuse light differently based on the observer’s point of view, or by changes in environmental lighting.
For example a crystal opal (a form of opal with a transparent body structure) will change its appearance drastically when viewed against different backgrounds as the primary source of light through the opal is changed by the surface the opal sits upon.
To summarise, our aim here is to identify several key elements that together combine to form the opal's unique play-of-colour attribute, allowing us to then assign a value per carat to the gemstone.
How To View The Opal
It’s best to start by placing the subject opal flat on a colour neutral surface (where able)
Preferably on a white coloured surface so the body tone is easier to discern, but make concessions as circumstances permit.
The idea here is to place the opal in a position that the facing we believe to be most appealing is pointing directly up away from the surface we have sat it on for viewing. So some effort must be made to determine which facing of the subject opal this should be.
Colouration of the Opal
We’ll start with arguably the most obvious facet: the opal’s primary colouration.
What I refer to here is the most predominant colour or colours when viewing the opal directly on its 'upward' facing surface, as opal changes colouration based on viewing angle it’s important that we view the opal and pick which direction this 'upward' face will be.
In general, priority goes to red spectrum colours as they are more valued than green spectrum colours which in turn are more valued than blue spectrum colours. So a predominately red/orange colour display is going to be more 'expensive' than a predominately green colour display.
Once you have selected the upward face of the opal, the predominant colour of that face can then be used as the primary value indicator. That is to say the colour that is visible in greatest quantity and consistancy, as the goal is to determine a kind of 'standard' to assess the entie opal by.
Opal Colour Vibrancy
Secondarily to the colour is the vibrancy – the strength of the colouration within the opal. Strong bright colours are far more important than dull ones – this is part of why black opal is so sought after - as the darker body tones tend to improve the colour contrast of opals, making for more apparent colourations.
Viewing angles matter here, as from certain directions the opal’s colour palate will appear brightest – this is an important factor for determining the best settings for a particular opal, a gemstone that has a strong display of colour from directly in front (or above if we have the gemstone sat on a surface)
and weaker colourations from the sides of the opal would more often be set within a pendant - where the opal can be frequently seen from its strongest colour angle. (the front)
However, opals that display brightly from many directions would be better suited to be set within rings, where the viewing angle can be quite broad and more of the opal’s play-of-colour can be displayed to the onlooker.
Opals that present bright flashes of colour from multiple angles are often more highly valued as they have a wider range of uses in jewellery as they display a broader range of colour patterns.
Patterns within Opals
The pattern or patterns within opals begin to matter at this stage, as we’ve determined the primary colouration and strongest viewing angles – some patterns within th gemstone will serve to increase the colour contrast and in turn the opal’s apparent play-of-colour from a given direction, whereas in other opal gemstones the pattern can decrease contrast and reduce the play-of-colour.
While some value is placed upon the kind of pattern itself due to comparative rarity, (such as a harlequin pattern for example) largely the additional value often comes from the contribution(s) made to the individual opal’s play-of-colour.
Finally I’d like to expand upon some of the supplementary characteristics that while not necessarily primary considerations will greatly impact the general appearance and designation of the opal being considered.
First of these is the opal body tone scale, this scale refers to the level of contrast provided by the host structure’s foundational tone and extends through 9 distinct severity categories labelled from N1 to N9.
Black Opal Scales
Categories N1 through to N4 describe dark (almost black) colour tones, these rarer darker tones are collectively considered to be characteristics of black opal, Opals mined from lightning ridge due to higher concentrations of manganese within the hosting structure tend to fall into these categories.
But any opal of sufficiently dark body tone regardless of composition can be referred to as a black opal.
Dark Opal Scales
A Step along from a Black Opal is a Dark Opal, and these are encompassed by the tone severities of N5 and N6 and are perhaps better described as ‘gray’ body tones.
Light Opal Scales
Finally the larger and more commonly found range of opal body tone is the ‘light opal’ grouping from N7 to N9 with N7 being a milky beige to N9 being pure white.
In conjunction to body tone is body clarity – how transparent, translucent or opaque A gemstone’s body is. It’s by this measure that we distinguish between crystal and milky opal.
transparent opal categories would be referred to as crystal opal, and more translucent opals are considered to be of the milky opal category. With opaque opal types often (but not always) being boulder, matrix or milky variants.
How transparent the host stone is can have a large impact on the stone’s play-of-colour as light is more or less able to pass through the structure – this can wash out the stone’s colour or cause it to become more pronounced based upon the availability and direction of local lighting, in crystal and milky opal variations in particular the opal can appear to be a completely different colouration than is usually seen under controlled conditions (particularly if backlit).
Opal Carat Weight
And at last we have the Opal’s carat weight – a multiplicative factor of a given gemstone’s value.
As it is usually a measure of the size of the structure and not necessarily the quantity of precious opal, some care must be taken here. A large carat triplet opal gemstone has comparatively little precious opal for its carat weight compared to a natural opal gemstone of equivalent carat weight. (and is priced accordingly).
However for hypothetical’s sake we can also say that (As is never the case with opal) if all other factors of play-of-colour are equal, the larger stone is more highly priced compared to the smaller - as the price of opal is at its core: quality (play-of-colour) per carat.
If you would like to learn more about these facets we would suggest reading the nomenclature documentation as presented by the Gemmological Association of Australia to the Australian Gemstone Industry Council, which we have linked below through the international opal academy’s website.