An Opal is a gemstone that is chemically composed of silica and water. Its name derives from the Latin word “Opalus”, meaning “to see a change of colour”, and it is easy to see why it was so named, as opals display brilliant flashes of colour in varying degrees. This gemstone has a hardness of 6 on the Mohs scale.
Opals were greatly favoured gems during Roman times. Sometime around 77 BC, Pliny the Elder, in his treatise ‘Natural History’, not only credited it with being the most highly prized and valuable of all the stones, but also said:
“For in the opal you shall see the burning fire of the ruby, the glorious purple of amethyst, the green sea of the Emerald and all glittering together, mixed after an incredible manner.”
Up until the late 19th and early 20th centuries, opal was regarded as a stone of good fortune. After that, most probably for commercial reasons, opals were considered to be unlucky. That said, this beautiful mesmerising gemstone is still considered to be the October birthstone.
Where can you find Opal?
Although Opal can be found in a variety of places, since the 1980s Australia has dominated global opal production, accounting for around 95% of the global output. The first Opal blocks in Australia were found by chance on a cattle station called Tarravilla in 1849, and the first Opal digging started in 1890 at a place called White Cliff, mining Opal rocks. Opal of differing qualities occurs in many other countries, including Zambia, Ethiopia, Guatemala, Indonesia, the USA, Brazil, Mexico, Poland, Peru, Canada and New Zealand, to name a few.
How does opal form?
Although we are not entirely certain, the most widely held theory is that is opal is created by silica-laden fluids moving through layers of sub-surface strata, under hydraulic pressures caused by tectonic processes, at temperatures of more than 100°C. When these fluids cool down they harden and become opal.
Another theory is that rich aerobic soil bacteria and microbes conserve the opal by making the creation of acids and enzymes that form opal easier.
What types of opals are available on the market?
Precious opal, the most valuable and rare of which is the black opal. Its body colours can vary from dark grey to jet-black, and this is the generic term is given to any opal that has a dark body colour when viewed from above. An example of a black opal is the 25 carat Millennium, which had a “Harlequin” pattern, and was sold several years ago by R. W. Wise, Goldsmiths.
Light Opal has a body colour of cream to white, and a brilliant light gem can be very precious. Although it is a beautiful gem, the black opal is ten times more expensive.
Boulder opals are opals which form naturally within the host rock, the most expensive of which are those with a dark body tone. An example is the 2600 carat boulder opal known as “Galaxy”.
Translucent opals are usually known as water opals, although one exception is found in Mexico, which produces some translucent orange gems known as “fire opals”. These have a greater intensity of one colour rather than flashes of multiple colours.
How are the prices established and what determine their values?
In general, the richer and more pronounced the colours, the more valuable the opal, especially if the colours are displayed against a dark grey or black background as in a case of “black opal”. High-quality black opal prices can rival those of diamonds.
The colours are created as a result of diffracted light, and the dominant colour can significantly increase or decrease the value of an opal. Red and orange are the most highly prized, and a perfect combination would be a strong red play-of-colour on a dark body-toned black opal. In terms of value, after red and orange comes yellow, green, blue, indigo and finally violet.
Brilliance and body tone can also affect price. The degree of brightness or brilliance can make an opal more attractive because it will emphasise the colours that it displays, while an opal’s body tone is qualified by different levels of darkness. The most valuable one is N1 (the darkest), and the least valuable N9 (the lightest).
Finally, the fascinating patterns of a highly displayed diffracted colour can also increase the value of an opal. This is especially so of particular patterns such as a
“Harlequin”, which are unique and so have an enormous impact on price. “Pinfire” and other smaller models are not so desirable.
Other facts about opals
Due to its low hardness opals are usually cut as cabochons or flat ovals. Opal is very susceptible to changes in temperatures, and heat should be avoided, including photographic and display lights. Some less scrupulous dealers will sell the gemstone immersed in oil to disguise any cracking or crazing that may have occurred in the opal.
Opal doublets and triplets are opal composites formed by attaching a natural piece of opal to another material, and should be advertised as such. Opals can also be “treated” or enhanced by heating and soaking the stone in an oil or sugar solution and then burning it off or applying acid; this will make the opal appear darker.
It is also very common to find synthetic opal on the market, however a trained gemmologist should be able to tell the difference, the most important of which is the way the colours splashes are spaced. On a synthetic opal these will be very evenly spread out with clearly defined edges, and this phenomenon is often described as being like “lizard skin”. They are usually made from plastic, glass or latex.
*all images are from pinterest.com