Opal was considered highly valuable up until the 19th century and used to be prized by the European royalty. Nowadays, since the discovery of significant deposits in Australia, it has lost some of its former glory but remains a precious gemstone. It also remains an attractive choice for jewellery makers due to its unique visual properties. This gemstone comes not only in a range of colours from black to white, but also displays a variety of patterns.
Colour
Opal can be found in a variety of colours, and its visual characteristics generally cause it to display a multitude of colours. While not all opals exhibit this characteristic, a lot of them can be referred to as iridescent due to their multi-colour appearance. Highly rare varieties exhibit a distinct black hue. Most commonly, though, white and green are the colours that are most frequently found.
The colour variety is caused by its internal structure which causes it to diffract light.
One of the more important aspects of an opal's colour is its pattern. While most opals offer a degree of iridescence, they also do so in different patterns.
Pattern
There are quite a few known patterns which opal gemstones may exhibit. Some can be quite elaborate and striking, while others are simpler and less clear.
In general, patterns that show more clear and vibrant colours are more desirable. Some of these patterns include Harlequin, Chinese Writing and Floral
. Especially the former, Harlequin, is quite rare and sought after. The Chinese Writing pattern's name comes from its apparent resemblance of Chinese characters.In contrast, simpler and less clear patterns such as Moss or Grass
tend not to attract as much attention, and are also more common which generally makes them less desirable.Characteristics
Opal varies in its opacity: from opaque to semi-transparent. In terms of hardness, it is usually rated between 5.5 to 6 on the Mohs hardness scale, making it suitable for jewellery crafting but it is not an overly hard gemstone.Its internal structure diffracts light, which brings about the multicolour appearance.
Synthetic Opal
Artificial opal stones do exist, however most of them would be more appropriately named imitations: they tend to contain plastic additives, and appear to be porous unlike the natural gemstone. In addition, they are not fluorescent when viewed under ultraviolet light which further differentiates them from natural Opal stones.
Opal Doublets
These stones tend to be made of two layers adhered together: a thin layer of opal, and a layer of backing such as colourless opal or black industrial glass. If the opal stone is thick enough, usually its edges are rounded off to give it a cabochon (domed top) shape.
Opal Triplets
Triplets tend to be made of a layer of backing, an extremely thin layer of opal, and a layer of clear glass, quartz, or plastic capping shaped into a dome. Triplets tend to be easily identifiable as the top layer – usually glass – reflects light differently than actual opal. They can also be identified by looking at the side of the stone – a triplet will usually show a clear straight line on the side indicating that it is made of several layers as opposed to being a solid stone.
Value
An opal's value depends on many things. Its colour, pattern, shape and size can all affect it as will the cut. In particular, black opals can be extremely valuable due to their rarity. In contrast, white opals usually don't command a high price. Darker coloured opals tend to be valued more as their darker surface allows them to display their brilliant colours more clearly and vibrantly than their lighter counterparts.
When it comes to doublets and triplets, they are usually less expensive than a solid opal gemstone of comparable characteristics as doublets and triplets are made using a substantially thinner layer of opal matched with other – typically non-gemstone – materials. This is especially true for triplets, which contain a layer of opal even thinner than doublets.
It has to be noted, that valuing an opal is often not a clear-cut, simple process driven by set standards and guidelines. Due to its nature as a gemstone exhibiting various patterns, an opal's value may be subjective to a degree. Even if the stone's visual attributes such as cut, size, quality, etc. can be measured to a certain standard, doing the same for its pattern can prove more challenging.
Unlike precious opal, common opal is generally considered to be of little value.
Sources
Opal deposits may not be limited to Australia, but it is there that most abundant deposits of this gemstone are found. Therefore, it is of no surprise that Opal is Australia's national gemstone.
It is worth noting at this point, that it is the precious opal deposits that the country is known for. Common opal, which is not of particular interest as a gemstone, can be found throughout the rest of the world.