Birthstone Guide: Opal for Those Born in October

With an iridescent radiance, the captivating opal is the birthstone for October. Here, Lily Faber FGA DGA EG, explores the history and properties of this fascinating gemstone.

Opals have been valued for millennia, even before Ancient Greek and Roman times. There are two main types of opal: precious and common. The precious variety shows that magical play-of-colour which is so highly sought-after and can capture the attention and imagination of anyone who lays eyes on one of these gemstones. The more common variety simply does not show this optical effect, but can come in a variety of colours from pink to green to blue to yellow. Another variety is known as fire opal, which is a transparent to translucent variety that is orange, red or yellow and sometimes displays play-of-colour, but often does not.


Precious Opal, Image Credit: Pat Daly

Believed by the Greeks to give one the power of prophecy and foresight, opals enjoyed a long period of favour. The Romans thought that opals represented purity and hope. Opals were regarded in high esteem until the 18th and 19th centuries when the perception changed and they were believed to be unlucky, causing misfortune and bringing harm to the wearer.


Common Opal, Image Credit Pat Daly

Prince Albert gave Queen Victoria the Oriental Circlet Tiara that featured 11 precious opals. These were later replaced with rubies by her granddaughter Queen Alexandra, who believed them unlucky.

 Opal with Striated Colour Patches. Image Credit: Pat Daly 

Prized precious opals were relatively rare prior to the 19th century, the best examples coming from present-day Slovakia. Today, there are many localities where opals can be found, but the best and most valuable ones were discovered in the late 19th century in Australia. Places of highly desirable opals include deposits in New South Wales called Lightning Ridge and White Cliffs. 

‘Synthetics’
There are many natural opals on the market, but there are also manmade ‘synthetic’ opals. First created by Pierre Gilson in 1974, these opals are very close to matching the physical and chemical structure of a natural opal. Synthetic is in quotations because it is not completely identical to its natural counterpart. It is essential that one must be able to distinguish between these manmade opals and the naturally forming ones when going to purchase an opal. This can be done with observation using a 10x loupe.

Natural opals with play-of-colour have irregular patches of rainbow hues that flash at many different levels within the stone as it is turned. The patches of colour are irregularly shaped and can be limited in the colours shown depending on the inner structure of the opal.


Synthetic Opal - Columnar Structure.
Image Credit, Gem-A, Pat Daly

‘Synthetic’ Gilson opals still display play-of-colour, but it can be much brighter and more consistently shaped and displayed throughout the stone than natural opals. Additionally, these patches of colour have a polygonal outline to them, giving a ‘lizard skin’ appearance. If cut as a cabochon, there will also be a columnar structure to the patches of colour on the side of the cabochon, which is completely unlike natural opals.

Care and Caution
Opals contain up to 30% water, which means that they are susceptible to drying out when exposed to heat. This can cause crazing, or cracking, that is irreversible. When storing your opals, always make sure that it is in a cooler temperature, preferably with a bit of moisture in the air (a small dish of water or cotton ball will do). Opals are also porous, and can be easily damaged by acids and chemicals such as detergents, perfumes and jewellery cleaner. Finally, opals are soft with a hardness of 6 on the Mohs scale. This makes them vulnerable to knocks, scuffs and abrasions, which means they are more suited for earring and necklace settings rather than rings in order to keep them safe while wearing. 

Opal Boulder, Image Credit: Gem-A

Today, opals are enjoying resurgence in popularity, despite the misgivings that they can bring back luck to those who wear them. For those born in October, opals can and should be considered a bit of gem magic to be enjoyed for their stunning play-of-colour that is unmatched by any other gemstone.

Interested in finding out more about gemmology? Sign-up to one of Gem-A's courses or workshops.

If you would like to subscribe to Gems&Jewellery and The Journal of Gemmology please visit Membership.

Cover image: Precious Opal Matrix. Image Credit: Pat Daly. 


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