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Gemstones












Above: Hemmerle; Andreoli; Glenn Spiro Below: Chanel; Moiseikin
Above: Hemmerle; Andreoli; Glenn Spiro Below: Chanel; Moiseikin

Garnets Part II: Grossular, Andradite, Uvarovite

Compared with the well-known reddish browns and purplish red garnet varieties of pyrope, almandine, and spessartine, the second solid solution series producing gem quality garnets generally goes under the radar of your average jewellery customers.

This series comprises three species – grossular (calcium aluminium silicate), uvarovite (calcium chromium silicate), and andradite (calcium ferric-iron silicate) – all of which can exist together in the one stone in varying amounts.

Garnet varieties within this series that are better known to the trade and to avid jewellery fans include the vibrant green tsavorites and demantoids.

Tsavorite garnets were first discovered in Tanzania, closely followed by localities found in Kenya. Hence, they were so named after the Tsavo National Park near the border of Kenya and Tanzania, by the deposit's founder geologist Campbell R. Bridges, in collaboration with Tiffany & Co.

Among the localities following this same tsavorite-bearing geological formation discovered by Bridges is the Scorpion mine in south-eastern Kenya.

This mine is famous for producing incredible gem-quality tsavorite ideal for the jewellery industry, as well as beautifully formed gem-quality crystals valued by mineral specimen connoisseurs.

Tsavorite is a variety of grossular garnet, coloured a striking green by the presence of chromium, vanadium, and even iron as trace elements. Gem quality stones are appreciated for their rarity and are highly valued by collectors.

Although both are a vivid, eye-catching green, tsavorite is not to be confused with the andradite variety of demantoid garnet.

Popular and captivating, demantoid garnets are valued not only for their colour, but also their dispersion.

This notably strong quality produces an impressive fire capable of rivalling even diamond - hence the name demantoid, meaning ‘diamond-like’.

The variations of green seen in demantoid vary according to locality. A secondary colour is often present, such as yellow, brown, or rarely blue. Although it may be the result of a few possible factors, their colour is often due to chromium occurring as a trace element or iron.

The first discovery of demantoid, and the most famous of localities, was in the Ural

Mountains of Russia. Specimens from Russia are treasured for their gem quality, fantastic colours, and the quality of the intriguing fibrous asbestos inclusion termed ‘horsetail’.

Once thought to be an inclusion exclusive to demantoid, horsetails have since been found in other andradite garnets that are completely absent of any green hue, and that therefore do not qualify as demantoid variety.

The inclusion is indicative of the serpentinite geological setting in which these gems form.

Though Russia is the most famous locality, demantoid garnets have been found in Italy, Iran, Namibia, Madagascar, and Mexico. Other varieties of andradite garnet include melanite (black) and topazolite (yellowish-brown).

Sharing the same species as tsavorite, hessonite is another variety of grossular garnet.

This yellowish-brown to reddish-orange gem is a lesser-known material, with large and high-quality pieces commanding higher prices.

Other notable varieties include grossular garnets rich in chromium as a trace element, with a colour-shift from yellow-green in daylight to orange-brown in incandescent light from Ethiopia, and the grossular-andradite garnets of greens, brownish, and yellowish colours known in the trade as Mali garnets.

Mali garnets have been gaining more popularity in recent years, desired for their attractive colours and high dispersion.

The country of Mali in West Africa, after which they are named, remains the only known source of these gems.

With a growing appreciation in the trade for gemstones of vibrant colour and optical performance, garnets are among the previously disregarded gems that are now enjoying more of the spotlight in the boutiques of jewellery ateliers.


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ABOUT THE AUTHOR
Mikaelah Egan

Contributor • GAA Editorial


Mikaelah Egan FGAA Dip DT began her career in the industry at Diamonds of Distinction in 2015. She now balances her role at the Gemmological Association of Australia with studying geology at the University of Queensland. Visit instagram.com/mikaelah.egan For more information on gems and gemmology ,go to www.gem.org.au

Ellendale Diamonds Australia
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