October Birthstone – Opal
October actually has two birthstone but for this blog post we’ll look more in depth into the Opal. In ancient times it was known as the Queen of Gems because it encompassed the colours of all other gems. Each Opal is truly one-of-a-kind; as unique as our fingerprints.
Opal derives its name from the Greek word Opallios, which means ‘a change of colour’. Other sources indicate that the term was derived from the Sanskrit word Upala meaning stone. Australian aborigines, meanwhile, believed that the creator came to earth on a rainbow, leaving these colourful stones where his feet touched the ground. According to some thoughts, the Opal was believed to make its wearer invisible making it a popular talisman with spies and thieves. The stone has a unique property in changing colour, believed to indicate the health and mental alertness and fitness of the wearer. This October birthstone colour is associated with purity, innocence, hope and faith.
Opals are actually formed by the rain. Seasonal rains soaked the parched outback, carrying silica deposits underground into cracks between layers of rock. When the water evaporated, these deposits formed opal. Sometimes, silica seeped into spaces around wood, seashells and skeletons, resulting in opalized fossils.
Australia produces around 95 percent of the world’s supply, the majority of white is the white Opal from the fields of Coober Pedy in South Australia. However, Opal is also mined in Mexico, Brazil, Honduras, Ethiopia, the Czech Republic and parts of the U.S. Opals have even been found on Mars! And is one of only a handful of gemstones that have ever been discovered outside of our planet (the other being Peridot from outer space).
Opal’s characteristic “play-of-colour” was explained in the 1960s, when scientists discovered that it is composed of microscopic silica spheres that diffract light to display various colours of the rainbow. These flashy gems are called “precious opals;” while those without play-of-colour are “common opals.” After scientists discovered the spherical silica structure of opal, they figured out how to synthesise it in 1974.
How to Buy Opal
Like diamonds, opals can be evaluated by colour, clarity, cut and carat weight. But these unique gems also have several additional conditions to consider.
Colour – Colour is the key factor of opal quality: both the background “body colour” and the flashing “play-of-colour.” Dark backgrounds provide more contrast against vivid play-of-colour, making black opal more highly valued than milky white varieties. This Play-Of-Colour is measured on a brightness scale of 1 to 5, from faint to brilliant. Warm colours like red and orange are generally rarer and more valuable than common blues and greens, although colour range and coverage also play a role.
Pattern – Pattern is another factor unique to opal. Descriptive names like stained glass, peacock, rolling fire and Chinese writing distinguish opal patterns. Gemologists typically prefer large, concentrated patches over small specks.
Clarity – Different opal varieties have varying clarity standards. Crystal opals should be transparent, while opacity makes black opals more valuable. A cloudy, milky haze lowers any opal’s value, and may indicate instability.
Cut – Fine opals are often cut into irregular shapes to emphasise play-of-colour. When possible, opals should be cut cabochon with rounded domes. But most opal comes in thin layers, which are commonly mounted on another dark stone like onyx or obsidian (as a doublet) and sometimes capped with clear glass or plastic (as a triplet) to make this fragile gem more wearable.
Opals may be treated with wax, oil, smoke, plastic or other additives to enhance lustre. Identifying enhancements or synthetic materials may require specialised lab equipment, so it’s best to work with a jeweller who understands the criteria that determine opal’s value.
Due to it’s high water content, opals can easily crack or “craze” under extreme temperatures, dehydration or direct light. Crazed opal sells at much lower prices, and is more susceptible to fracture. Even high-quality opal demands delicate care.