Opal is an easy-to-recognise gemstone that has enchanted anyone that comes across with its beautiful display of colours. It is the only stone that displays the phenomenon, play-of-colour (POC), through its unique crystal structure. Precious opal is capable of displaying every colour of the spectrum, and it truly defines the definition of “no two gemstones are the same” from the result of its uniquely-constructed POC patterns. For its popularity, it is commonly recognised as the birthstone for October.
Opal has a simple chemical composition of hydrated silica oxide with variable amounts of water, but the formation of opal is completely different from typical crystalline gemstones. For opals to form, it involves evaporation of silica-rich solutions between layers of sedimentary rocks over millions of years. This process repeats many times during its formation period, where seasonal droughts and floods repeatedly stack silica layers upon layers until it opalises. Opalisation can occur in any organic or non-organic cracks or space. Only well formed opals can display POC.
Unlike the majority of gemstones, opal does not have a crystal structure, which makes it an amorphous gem. Its atomic structure consists of billions of submicroscopic silica spheres that vary in size and arrangement. What determines the size and arrangement of those silica spheres depends on the geological conditions. Different weather patterns and evaporation rates are known causes for the variations. In the end, it affects the gems ability to display POC altogether.
Opal is relatively softer than many other gemstones at 5.5 – 6 on the Mohs hardness scale. Thus, opals are usually placed in bezel settings rather than with prongs to provide more all-rounded protection to the gem. Another unique characteristic of this gem is that it contains water. Typically it contains 3% – 10% water content, but some varieties as much as 20%. That high water content does not affect opals at all as it is part of its chemical composition. On top of that, some opals are capable of absorbing water due to its porosity. These are known as hydrophane opals, mostly mined from Ethiopia.
Opal mining started early in history, dated around 400BC. Slovakia was known to be the first source for ancient Europe. That point of time, opal mines in modern-day Slovakia were part of Hungary. Therefore, many opals are dubbed “Hungarian” opals, and it is not changing to “Slovakian” opals anytime soon. In another part of the world, Mexico was the source of the New World. During the reign of the Aztecs in Mexico, the people used opals for burial and worship ceremonies. When the New World was conquered by the Spanish Conquistadors, Mexican opals were brought back to Europe along with other treasures.
Australia soon became the top opal producing country in the late 1800s with many deposits discovered throughout the century. Near the 1900s, Australian white opals began to appear in the world’s market and directly compete with “Hungarian” opals. With Australian opal bearing never-before-seen POC quality, European opals were no match that are soon operation ceased. Aside from white opals, the discovery of black opals in Lightning Ridge made headlines across the world. Black body tone of the opal gave an extraordinary appearance and contrast to the POC.
More deposits continue to emerge in the 1900s, locations such as Lightning Ridge (1905), Coober Pedy (1912), Andamooka (1930), and Mintabie (1978). Until today, Australia produces over 95% of black and white opals for the world market.
Ethiopia is the latest country to produce opals in large quantities. Like Australia, deposits were discovered in different decades starting from 1994, followed by 2008 and 2013. It can be known as “Welo opal”, and it can match the beauty of Australian products with lesser cost and larger. Now this country has become an opal heavyweight, only second to Australia. Other significant sources for white and fire opals are Brazil and Mexico respectively, along with few minor producers like Honduras, western part of USA, and Russia.
As mentioned above, well-formed opals are capable of displaying POC. Its silica’s structure is the fundamental factor for an opal to be phenomenal. How it occurs is how entering light interacts with the opal’s orderly arrangements of silica spheres. When light strikes the opal surface, it travels through the spheres, diffracts, and breaks up into its spectral colours. Small spheres produce blue which is the most common, whereas larger spheres produce red which is the rarest colour of POC.
The possibilities of POC patterns are endless, and there are no ways to predict how it will be. Among all, there are three main terms to describe the patterns: Flash, Pinfire, and Harlequin.
- Flash: consists of large areas of POC. Can display be in any colours, either in one single hue or several hues.
- Pinfire: very small patches or dots that are set close together. Considered the least valuable of all POC patterns.
- Harlequin: mosaic-like patterns of different colours. Usually rectangular patches with edges touching each other. This rare pattern is unique to black opals and it is a collector’s type of pattern.
There are many types of opal in its existence. Some dazzle with POC, some do not. For a common opal, also known as potch opal. If it does not diffract light, it has no POC. Despite no POC, it has a wide range of attracting body colours and inclusions that grant common opals unique features such as pink opals, and moss or dendritic opals.
Opal’s background colour is what sets any type of opals apart. Background colour of opals is as important as any gemstone’s colour and it can happen to be almost any colour. Majority of the precious opals in the market are made up of white opals. Other than white, slightly grey background colour is also considered to be white opals. Black opal, on the other hand, has a dark grey to black background colour. It is considered the most valuable, desirable, and rarest among opals which can be sold for high prices. That is due to black body colour providing a distinct contrast for the POC to stand out.
Other significant opal types include boulder opal, crystal and water opal, fire opal, and assembled opal. The name ‘boulder’ may sound as if a huge mass of opal, but it is thin layers of opals included in its original matrix as a natural reinforcement. Matrix may vary either sandstone or ironstone. Boulder opal can look unusually unique with dark brownish matrix and POC seeming through its cracks and crevices. Crystal and water opals are transparent to semi-transparent opals. The main distinguishing factor for both is the intensity of its POC. Crystal opal shows exceptional POC while water, or jelly opal shows faint and small portions of POC.
Another familiar variety is the fire opal. Like any other opals, fire opal can be identified by its background colour. Typically, this variety shows yellow, orange, or red with transparent to translucent body. It may or may not have POC and occasionally faceted as well. For a long time, Mexico has been producing the majority of fire opal, and for that reason, fire opal is widely known as Mexican opal. Fragile opals that are not thick enough to be set in jewellery without breaking will be assembled with an additional backing. Assembled opals can be a doublet or a triplet depends on the number of materials introduced with the opal. The backing is often composed of obsidian, black chalcedony, glass, or plastic. As for a triplet is an opal sandwiched by a black backing and a colourless domed top of either quartz or glass.
Opals are increasingly collectible be it for its value or its beauty. Speaking of beauty, evaluating opals are highly subjective. The quality of opal rests on many factors, for example, its type, quality of POC, symmetry of cut, and sometimes clarity too. Anyhow, the POC is the main attraction and requires to be fully appreciated. To do so, rotate the opal 360 degrees to obtain every spot of POC on the gem. Finally, based on the account of an Australian tourist, prices of black opals in Australia are getting ridiculous prices as of early 2020 due to its scarcity. Rarity combined with great demand for black variety has caused its value to sky-rocketed.
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