How to Value Opals

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. Unlike the standardised valuation of diamonds, opal valuation can be a more subjective topic. The type of Opal, the Opal’s presented pattern, body tone and colour play are all factors which are considered when valuing an opal gemstone.

1. Play of Colour

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 silica 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.

2. Colouration of the Opal

The colours present in an opal is an important factor when considering its value.

In general, red spectrum colours are the most valuable, followed by green spectrum colours which in turn are more valued than blue spectrum colours. For example, a predominately red/orange colour display is going to be more valuable than a predominately green colour display.

3. Vibrancy of the Opal

Secondary to the colour of an opal 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 when determining the best setting for a particular opal. For example, an opal 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 be better suited to 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. This is because they have a wider range of uses in jewellery, as they display a broader range of colour patterns.

4. Opal Patterns

The patterns within an opal are another consideration when valuing opals. Certain patterns can enhance the colour contrast and amplify the opal’s play-of-colour from specific angles, while others might reduce this effect. Highly sought-after patterns, like the rare harlequin pattern, significantly increase an opal's value due to their rarity and visual impact. Other notable patterns include the pinfire, floral and ribbon patterns, each adding unique characteristics to the opal’s appearance and desirability. Ultimately, the value of an opal is often tied to how its pattern influences its play-of-colour.

Additional Factors in Opal Valuation

Beyond the primary considerations, several supplementary characteristics greatly impact the general appearance and designation of an opal.

Opal Body Tone Scale

The opal body tone scale, ranging from N1 to N9, measures the contrast provided by the host structure’s foundational tone.

  • Black Opal Scales (N1 to N4): These darker tones, often found in opals from Lightning Ridge due to higher manganese concentrations, are classified as black opals. Any opal with a sufficiently dark body tone can be referred to as a black opal.
  • Dark Opal Scales (N5 and N6): Known as dark opals, these tones are better described as ‘grey’ body tones.
  • Light Opal Scales (N7 to N9): This range, from milky beige (N7) to pure white (N9), encompasses the more commonly found light opals.

Body Clarity

Body clarity, which refers to the transparency, translucency, or opacity of an opal, is another important factor. This clarity affects how the gemstone's play-of-colour is perceived.

  • Transparent Opals: These are referred to as crystal opals.
  • Translucent Opals: These are considered milky opals.
  • Opaque Opals: Often, but not always, these are boulder, matrix, or milky variants.

The transparency of the host stone can significantly influence the stone’s play-of-colour. In crystal and milky opals, the stone’s appearance can vary dramatically under different lighting conditions, potentially changing the perceived colouration.

Understanding these additional factors is essential for accurately valuing and appreciating the unique beauty of each opal.

Opal Carat Weight

When considering opal carat weight, it is essential to understand that it measures the size of the structure, not necessarily the quantity of precious opal. For instance, a large carat triplet opal gemstone contains comparatively less precious opal than a natural opal of the same carat weight and is priced accordingly.

Hypothetically, if all factors of play-of-colour are equal, a larger stone will be more highly priced than a smaller one. This is because the core determinant of an opal's price is the quality of its play-of-colour per carat.

Valuing an opal is complex due to the various factors involved, such as play-of-colour, pattern, body tone, and clarity. An accurate valuation requires the expertise of an experienced opal evaluator.