Gem of the Month: October

Opal is a captivating gemstone. It is not classified as a true crystal but rather as a mineraloid, composed of microscopic silica spheres and water. This structure gives some opal, dubbed precious opal, its signature iridescence, also known as ‘play of color’. Common opal lacks this optical phenomenon, though its structure is basically the same.

The name “opal” is thought to trace back to the Sanskrit word “upala” (उपल), meaning “jewel,” and adopted by the Greeks in the word “opállios” (ὀπάλλιος). Though there are other suggested origins, this one holds more credence, as opals were a prized gem for thousands of years but the word opállios only became common in Rome around 250BC after an uptick in trade from Southeast Asia.

Opals can occur in nearly any base color, with white, gray, and green being most common and black opal the rarest. Opal can also occur in fossils, where silica fills the space left by something organic. As with most opal, precious opal fossils are comparatively rare, but common enough that most rock shops have a couple nice specimens.

The largest known precious opal, weighing approximately 55,000 carats, was found in Australia by Stuart Hughes. Truly massive by any account, but especially when you consider that the previous title holder was 11000ct, and the largest cut opal is only 3700ct. 

Opal is relatively soft and delicate so it is not generally suited to daily wear without extra caution. Their composition makes them sensitive to heat, chemicals, and abrasion. Their play of color depends partly on the water trapped within the stone so, to prevent the loss of this water, some opals are sealed or stabilized, though high quality opal usually doesn’t need this treatment. Rough opals are commonly stored in water or light mineral oil before being polished to preserve their moisture content, and to enhance their transparency so the cutter can see further into the stone.

When cleaning opals, it’s best to use a soft brush or cloth with warm water and mild dish soap. Ultrasonic and steam cleaners should never be used and soaking should be avoided, especially for composite opals like doublets and triplets. When storing opals, keep them separated from other jewelry, ideally wrapped in a soft cotton cloth. For long term storage, opals should be kept somewhere with a stable temperature and humidity, ideally around 70°F between 40% and 60% relative humidity, to prevent cracking or the play of colors to fade.