Although once known as 'bad luck', opals are fast becoming one of the most sought after gemstones in jewellery collections. But what makes one opal worth thousands and others mere pounds and pence? Here, Gem-A Instruments manager, Samatha Lloyd FGA EG, offers a quick but essential guide to opal value factors and what distinguishes a fantastic specimen from an average one.
COLOURS IN OPAL
Opal is composed of uniform spheres of silica, which form a grid-like structure. The spaces between these spheres contain a silica solution. When light passes through the spheres and hits the silica solution, it is diffracted, producing differing rainbow hues.
Colour play depends on the size of the spheres, for example, smaller spheres result in blue colours, but orange and red will be present when they are larger. The more uniform the grid-like structure, the more intense the colours will appear.
TYPES OF OPAL
Customers may be most familiar with light opal, which makes up the majority of mined opal. It has base colours that range from white, to milky white and light grey, with varying degrees of colour play dancing on top.
If the body of the opal is transparent – also known as light crystal opal - the colour patches can be seen below the surface. It is these specimens that command exceptionally high prices. Your customers may also be familiar with black, or dark, opal, which has a dark body colour - sometimes enhancing the brilliance of the colours. This is the rarest and most valuable opal variety.
There are a number of factors that alter the value of opals. As mentioned, black opal can command higher price points than light opal (especially with an inky black body tone), although this is not to say that a fantastic light opal cannot be more expensive than a mediocre black opal.
The brightness and brilliance of an opal is particularly important for its value, even if it is average in other areas. Therefore, lots of colours flashing on a dull stone may not command the same value as a gem with a higher degree of brilliance.
We have already hinted at transparency, but this is also an important value factor. Light opal is much more desirable if it is transparent, with crystal opals with vibrant colours being particularly prized.
A secondary and more complex layer of value arises when considering colours. The ‘dominant colour’ in an opal can affect its value, with red commanding the highest cost, followed by orange, yellow, green, blue, indigo and violet. However, do not forget, a blue-green brilliant stone will be more valuable than a dull red.
In some rare cases, the diffraction of light within an opal can cause interesting patterns to arise. These patterns can increase the value of a stone. ‘Pinfire’ and small dot-like patterns are less desirable than bold ones, such as stripes, peacock feather shapes and broad brushstroke-like flashes of colour.
ETHIOPIAN VS AUSTRALIAN
Customers may ask why one opal costs thousands of pounds, while another is mere hundreds or less. The answer could lie in its origin. Australia is a phenomenal source of opals and produces some of the world’s most incredible specimens.
Crucially, opals from this region have a lower water content, which means they are less susceptible to drying-out and less likely to exhibit ‘crazing’ - hairline fractures that impact the durability of the stone.
In contrast, Ethiopia is a newer source of opals, but some material has been found to have a much higher water content, making it unreliable and potentially unsuitable for jewellery.
With thanks to Hatton Garden-based gemstone supplier, Marcus McCallum FGA, for taking these striking photos of Ethiopian and Australian opals.
Read more facts in The Opal Story by Andrew and Damien Cody, available in the Gem-A library. ■
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Cover image example of Australian opal. All images courtesy of Marcus McCallum.
Our Heritage Series returns to celebrate two of Gem-A's most distinguished Presidents, father and son duo Sir William Henry Bragg and Sir Lawrence Bragg, who were jointly awarded the Nobel Prize in Physics in 1915 for their research in X-ray Crystallography.