August is here and with it our latest birthstone guide. Here, Lily Faber FGA DGA EG, explores August's spinel as we enter high summer.
Spinel is a relative newcomer to the list of birthstones and was added as an alternative to peridot for August’s birthstone by the Jewelers of America and the American Gem Trade Association (AGTA) only recently. It is known as the great imitator because it can look like so many different stones with its wide variety of colours.
Spinel Dreamscape™ 5.34 ct cut by John Dyer
Gem Courtesy of John Dyer & Co. Photo Credit: Lydia Dyer
Red spinels were commonly mistaken for rubies for millennia until the 18th century, when gemmology became a more established science, enabling us to differentiate similar-looking stones. Red spinels are considered important stones in British and other royal regalia, and are sometimes called ‘balas rubies’.
Spinel in Marble Matrix. Image by Pat Daly
Thought to protect the wearer from harm, spinels are also believed to take away sadness, replenish one’s energy and help overcome the challenges that life brings.
While red is one of the most popular colours of spinel, it can come in a large array of colours such as blue, grey, green, purple, orange and pink. One of the more intriguing colours on the market is a very bright pinkish –red spinel that looks like it is glowing bright, neon red. It is known as ‘jedi spinel’ because it glows like a lightsabre ala ‘Star Wars’, and was discovered in Myanmar in the early 2000s.
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Crystals and Inclusions
Spinels form either as octahedrons with flat, polished-looking faces that have a bright vitreous lustre, or they can form as spinel twins, which look like flat triangles with notches at the corners, known as re-entrant angles. Triangular etch pits can be found on the surface of these crystals.
Spinel Crystal, Triangular Growth Marks. Image by Pat Daly
These gemstones often look loupe clean with minimal inclusions. Some of the more common inclusions can be octahedral crystals that are a spinel-type mineral and are usually arranged in very neat rows. You can also see zircon haloes (circular stress fractures around a zircon crystal) and iron staining.
Feather Inclusion in Spinel. Image by Pat Daly
Synthetic spinel can be produced in a lab in virtually any colour. If produced by the verneuil flame fusion method, there are a few tests that you can do to differentiate between it and its natural counterpart. The refractive index of synthetic spinel is typically 1.727, which is higher than a natural spinel, which is usually around 1.718. Additionally, if you put the synthetic stone on the polariscope, you will most likely see a type of strain called tabby extinction, which looks like thin, shadowy stripes that move across the stone as it is turned. Finally, observe the stone and try to find any natural inclusions like the ones mentioned above. There are many natural spinels with minimal inclusions, but use additional tests to err on the side of caution.
Tabby Extinction in Synthetic Spinel. Image by Pat Daly
Care and Caution
Spinels have a hardness of 8 on the Mohs scale, meaning that they can resist scratching and abrasion more easily than softer gems that fall lower on the scale. As it has a very good stability with a high resistance to heat or chemical damage, spinel can be used in jewellery dips, but it is still advised that warm water and a mild soap are used to clean spinel-set jewellery. Avoid ultrasonic and steam cleaners for good practice.
With its range of colours, good clarity and bright vitreous lustre, it is time that spinel was more appreciated as a gem material in its own right. Even though it is a lesser known gemstone on the market, it is a worthy alternative to peridot as a birthstone for August.
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Cover image: Feather Inclusion in Spinel. Image by Pat Daly.
Did you think this might be emerald? You certainly wouldn't be the first to confuse the vibrant green of dioptase with the more recognisable, jewellery-lover's gemstone. Here, Gem-A assistant gemmology tutor, Dr Juliette Hibou FGA, explains more about the history, properties and origins of dioptase, an unusual collector's gem.