Opal and tourmaline are the dual birthstones for the month of October. Opal is also the stone associated with the fourteenth wedding anniversary. Ancient Arabian mythology asserts that opals hailed from the sky during electrical storms and the aboriginal peoples of Australia thought they were derived when a mystical creation visited mankind in a rainbow, leaving the stones as a colourful reminder of its visit. The Roman philosopher Pliny marvelled at the reflective qualities of opal and theorised they must have been created by volcanoes. Many cultures have revered these stones, believing them to bring prosperity and good fortune. The pattern and colouration found on each opal makes them unique, almost like a fingerprint, and it is this quality that makes them such desirable gifts. Black opals are some of the rarest specimens and they possess an unusual quality in that the colours they produce appear to be multi-layered on top of one another, often forming a floral pattern. When it comes to valuing an opal, it is the luminosity and colour of the stone that are the key determining factors as, unlike diamonds, clarity is not of great importance and inclusions are part of the very nature of the stone. The more colours that are present in the opal, the more sought after it becomes and to a lesser extent the pattern those colours form within the stone can also have a bearing. Opals possess a primary colour of either white or black (and each have their own dedicated followers). The ‘kaleidoscope’ effect of the stone’s secondary colours can then be observed upon these.
(An example of three Australian opals)
The full spectrum of colours produced by the diffraction of light through an opal derives from microscopic silica deep within the stone’s crystalline structure. Consequently, opals that contain microscopic silica are regarded as ‘precious’ opal (versus ‘common’ opal that do not). Surprisingly, most opal are made up of 6-10% water, giving them are relatively low 5.5-6.0 on the (1-10) Moh’s scale of ‘hardness.’ As a result, care must be taken when setting and mounting opal and the wearer must also be mindful of the detrimental effects of extreme temperatures and impacts with solid objects. However, if reasonable care is taken opal will last a lifetime and can be passed on to future generations to enjoy. Due to the amount of water contained within an opal it is important to ensure the stone does not dry out, as can be the case if it is placed in an environment containing dehumidifiers (such as a bank vault) for a protracted period. In this eventuality the stone can crack or ‘craze’ but a simple method to avoid this involves hydrating by immersion in water for a few hours every now and again. Australia, where 90% of the stones are found, remains the largest source of opal in the world, having been originally discovered there in the mid nineteenth century. These deposits originated when torrential rains percolated through the ground, forcing silica deposits through small fissures in the bedrock, which gradually evaporated and formed the stones over many years. More recently opal has been unearthed in the USA, Mexico, Brazil, Honduras, Mexico the Czech Republic and Ethiopia.
(An example of a blue tourmaline)
Tourmaline is a vibrant alternative October birthstone to opal. As with most coloured stones, its colour is determined by the presence of trace elements and they are found in the full spectrum of colour. For this reason tourmaline entered ancient Egyptian mythology as having passed through a rainbow within the earth before being won from the ground and the origins of the Sri Lankan name for the stone, “tura mali” translates to “stone of mixed colour.” Despite its ancient origins, the first documented discoveries occurred in the 1500s in what is modern day Brazil. Modern production takes place in Australia, Burma (Myanmar), Nepal, Russia, Sri Lanka, Afghanistan, Pakistan, Tanzania and the USA. Many cultures have attached mythical qualities to tourmaline, including the belief that it guards against poisoning and even wards off feelings of anxiety and depression. At a relatively robust 7.0-7.5 on the Moh’s scale it is fairly durable and suitable for everyday use. The majority of tourmaline found on the market have been subjected to some form of heat treatment. This enhances the colour of the stone, making its hue appear deeper and more vivid. Heat treatment slightly increases the value of the stone but buyers must ensure the treatment has been declared and it pays to read the laboratory report to ensure you are fully aware of the treatments it has undergone (as untreated examples with the same appearance will fetch a considerable additional premium). Buying tourmaline without the relevant paperwork and certificates is not advisable.
(The full spectrum of tourmaline)
Tourmaline are most commonly found in a rectangular cut due to the fact that this is the most stable shape and one that reflects and refracts light in the most desirable fashion. Nearly all tourmaline contain inclusions and it is the colour that is the primary driver of a stone’s value, akin to most other coloured gemstones. As a general rule, the whiter the hue of a specimen the lower its value. Red tourmaline are quite sought after and green examples represent an interesting alternative to emeralds. The exclusive Paraiba tourmaline of Brazil are by far the most valuable of all (see our earlier article: Paraiba tourmaline – the rare and enigmatic Brazilian gemstone). Due to the colourful native of tourmaline, they work well in combination with a wide array of other gemstones and many different jewellery styles and settings. The Chinese have been avid collectors of tourmaline for many centuries, so much so that the fall of the last Emperor in 1912 precipitated a worldwide collapse in the price of tourmaline. In recent years it has found its way into the collections of many fine Parisian and international jewellers, including Cartier, Van Cleef & Arpels, JAR, Bulgari and Boucheron. As Christmas fast approaches it is well worth considering the merits of gifting the one you love a fine tourmaline or opal, if they were fortunate enough to have been born in the month of October, which will forever be associated with these two vibrant gemstones.
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