This post is about opals, their sources, and what to look for in a good opal.
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Opals are among the most intriguing and visually stunning gemstones known to man. In ancient Rome, the opal symbolised love and hope. The Romans called the stone “Opalus”, meaning “precious stone”. Pliny, the ancient scholar, said, “Some opali carry such a play within them that they equal the deepest and richest colours of painters. Other simulate the flaming fire of burning sulphur”.
Opals are found in Australia; one of the most famous mines is the wonderfully named “Lightning Ridge”! It is a major source of black opal. There is also a mine at Coober Pedy, which comes from the aboriginal word “kupapiti”, meaning “white man in a hole”. There are also opal mines Ethiopia and Brazil.
A high grade opal will be vibrant and full of a kaleidoscope array of colours. They form when seasonal rains soak into dry earth. The showers soak into the parched ground, carrying silica downward. During dry periods, the water evaporates, leaving a deposit of silica in the cracks between rocks. This is opal. For this reason, opals have a very high moisture content, generally between 5-10%, though sometimes as high as 20%.
The flashing colour produced in opal is caused by light interacting with the internal structure of the silica. This rainbow effect is called “play-of-colour” and is one of the most important factor to look at when considering an opal. Certain colours are rarer than others- reds are the most unusual, though it is a matter of taste as to which you prefer!
There are many different types of opal, distinguished from one another by background colour. The most common types are
- Black opal – black to dark grey background colour, translucent, with play-of-colour. These opals command the highest prices, and fine examples are highly prized.
- White opal – white to grey background colour, is translucent when held up to light, shows play-of-colour. This category would make up the bulk of opals on the commercial market. Because of this, there is a wide variety of prices possible.
- Crystal opal – translucent when held up to light, has a vibrant play-of-colour
- Water opal – Translucent, with little or no play-of-colour; this opal is of less value than the other categories, as a general rule.
- Boulder opal – Includes some of the host rock in the finished gemstone, shows play-of-colour. The rock can be sandstone or ironstone. My personal favourite, the patchwork effect of the brown boulder with the vibrant opal is gorgeous!
- Fire opal – Background colour is reed/orange/yellow, but may or may not show play of-colour. Because of the lovely background colour, sometimes fire opals are faceted.
There are also synthetic and assembled opals available, of low value. Always get expert advice if in doubt, as these can sometimes be very convincing to the untrained eye! Sometimes opals are treated or enhanced; it is important to establish if this is the case.
Opal’s pattern is graded according to two main factors. They are pattern and clarity. Pattern refers to the arrangement of the play-of-colour. In some opals, there are small dots of colour, in others there are large flashes of colour, and finally there can be large distinct patches of colour, whose edges touch each other. These arrangements are called Pinfire, Flash and Harlequin. Harlequin opals tend to be the most sought after. How the play-of-colour is “balanced” across the opal is important too, as it is most desirable to find an opal with a pleasing arrangement of play-of-colour; for example, it is generally nicer to have play-of-colour evenly spread across the whole of the gemstone rather than in just one area. This idea is called “balanced composition”, and is a term borrowed from the fine arts, and gives an idea of the intention to judge how harmonious the effect of the play-of-colour is.
Clarity refers to the degree of transparency of an opal, and also to its freedom from inclusions (in the same way as we judge the clarity of a diamond). Depending on the nature of the opal (black opal, fire opal, etc.), the market prefers different degrees of transparency, but as a rule, the most preferable is whatever maximises the visual impact of the play-of-colour.
In addition, clarity takes account of the flaws that may be present in an opal. Given the way it forms, opal can have certain natural clarity characteristics, which may or may not influence its value, though having no flaws is almost always the best!
If opals are left in hot or dry conditions, the moisture in them can evaporate, which leads to a fine network of cracks forming. This is known as crazing, and cannot be repaired. The fractures spoil the beauty of the opal, and reduce its durability.