Opal is one of the world’s most phenomenal and beautiful gemstones; in colour, pattern and rarity. As such, many cultures have associated opals with supernatural powers. The Ancient Greeks believed opals bestowed the gift of prophecy and protection. Arabic legend says it falls from the heavens in flashes of lighting. Europeans consider the gem a symbol of hope, purity and truth.
Let me straight away dispel the myth they are unlucky; this superstition derives from a 19th Century novel by Walter Scott called Anne of Geieirstein, in the story Anne wore a magical opal that was touched by holy water, she falls ill and the next morning is gone, with a pile of ash in her place. In fact conversely throughout history opals have been considered magical because of their beautiful show of colours. Indeed they once thought to preserve the life and colour of blonde hair – so I need to wear my opals more often!
For me personally as they are the birthstone for October, the month my lovely Mum was born, they are source of endless fascination and appreciation. Let me explain more about them.
This is the term used to describe the vivid display of iridescent colours for which this gemstone is valued. Opal is composed of spheres of sub microscopic silica – imagine them stacked in a grid like formation like layers of ping-pong balls in a box. This structure breaks up light into the spectral colours of the rainbow. The colours seen depend on the size of the spheres. The larger and more ordered the spheres (around 0.2 microns) cause the greatest colour range to include red, which is at the end of the spectrum and therefore the most prized, the smaller spheres (around 0.1 microns) produce blue/violet which has the shortest wavelength.
Types of Precious Opal
Experts divide gem opals into many different categories, but four of the main types are
This is translucent to semi-translucent with a play of colour against a white, beige or grey body colour. White opals are the more readily available and still show a great colour-play.
Are translucent to opaque with a black or dark grey body colour that show off the play of colour to the greatest visual effect. The dark background of the opal absorbs the otherwise scattered white light in lighter varieties of opal, so the visual impact of the spectral colours is significantly enhanced. Black opals are amongst the most rare so can command the highest prices.
Are usually a vivid orange variety coloured by small trances of iron oxide, but can also be brown, yellow or red in body colour. Prized by the saturation of this intense colour this material doesn’t show a play of colour, it is also known as ‘Mexican Opal’, ‘gold opal’ or Sun opal’.
As the name suggests boulder opals are formed in thin veins in sedimentary ironstone boulders. They are translucent to opaque with play of colour shown off against light to dark background, they are incredibly beautiful. The host rock fragments are naturally bonded and therefore are part of the finished gem. Boulder opal is usually less expensive then black opal, but can be of the same quality.
Sources and locations
The most important source of fire opal is the state of Queretaro in Mexico, some white opal from brazil, but 90% of the world’s supply is mined in New South Wales, Australia, the best from Lightening Ridge. Boulder opals come from Queensland and are unique to Australia, In fact opal was proclaimed the national gemstone of Australia in 1993
Opal is found in around the Great Artesian Basin, mainland Australia, 140 millions year ago it was a giant inland sea. Silica rich sands around 30 million years ago turned, because of weathering and climatic changes, into a soluble form of silica which flowed into faults and fissures eventually solidifying into what we call today common opal, and in fairly rare cases into gem quality opal.
It is believed that this soluble silica replaced cavities in the mud left by bones, wood and a good deal of crustaceous forms (we still find opalised coloured shells, bones and wood today) this mud level is today called the ‘opal level’, ‘opal dirt’ or just ‘the level’, this partly explains the diversity of shapes and forms of rough opal.
Opals only have a hardness of 6 on Moh’s scale (this is the scale of hardness used to denote the resistance to scratching in gemstones, from 1 – 10 (diamonds) anything below 7 (same as quartz) is considered relatively soft). Composed of silica and 10% water, great care must be taken with opals not to expose them to temperature extremes as the heat can cause the water to expand and contract and subsequently potentially craze and crack the opals. (This has contributed to the old wives tale they are unlucky) They should therefore be kept in a cool, dark place away from sunlight and heaters with a small damp ball of cotton wool to prevent dehydration.
Choosing your opal
Due to their amorphous structure most opals will be fashioned as a cabochon, fire opals can be faceted. This gemstone is all about the play of colour, should you wish to choose the finest and rarest, a black opal flashed with red is the most rare and valued. For me however, as every opal is unique, so is the opportunity to source the perfect stone for you based on form and aesthetics.
Beware of imitations and treatments
Opals are rare and desirable, as such can be imitated or treated. Some types of opal are stained to produce a darker background to show off the play of colour to better effect. The most. Pieces of opals that are too thin to be used alone often become part of opal ‘doublets’ or ‘triplets’. In these assembled stones a slice of opal is secured (usually with a dark adhesive) to a backing such as chalcedony, glass or plastic. A doublet (two parts) is made of the opal and backing, whereas the triplet (three parts) has a layer of colourless glass or rock crystal quartz on top.
A source you can trust
I am honoured to know renowned gemstone expert Duncan McLauchlan, with over 35 year’s experience in the industry he spent part of his career mining for opals in Lighting Ridge Australia where he continues fantastic working relationships. It is Duncan’s expertise and values I so admire, he is a also board member of the International Coloured Gemstone Association. Through Duncan we source only the finest and most beautiful opals directly from source to ensure their origin and integrity.