December Birthstone: Tanzanite

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Tanzanite is the trade name for the blue to violetish variety of Zoisite. The composition consists of mainly Silica, Calcium and Aluminium, with Vanadium as it’s colour causing element. Although it does occur naturally in its blue/violet colour due to natural heat during formation, 95% of blue Tanzanite’s for sale today owe their colour to heat treatment. Typically, untreated stones are brownish, and like many other gems with grey and brown tones, they’re subject to heat treatment. The focus of heat treatment is to remove these less attractive colours.

Tanzanite joined the original birthstones for December, Zircon and Turquoise in 2002. Interestingly, Tanzanite sits at 6-7 on Mohs Hardness Scale. With Tanzanite being a softer gemstone than others like Sapphire or Quartz, care should be taken when wearing it, as heavy wear will result in scratching or chipping of the stone and susceptibility to thermal shock can cause cleavage to form in stones.

 

First discovered in a region of Africa at the foot of the majestic Mount Kilimanjaro in 1962. The unique conditions for forming this particular gem occurs only in a 20-square-mile area, where it comes from in Northern Tanzania. First discovery of this rare transparent blue gem was thought to be a new source of highly transparent Sapphire with Pleochroism. Interest within the gem world and the public was instantaneous. Rough stones flowed freely from the mines and theft was rampant. Tiffany & Co stepped in, in 1968 to promote the new stone and gave it a name as exotic as its origin. Gemmologists by 1969 had in fact identified it as a new transparent variety of Zoisite and named it Tanzanite. Previously only known at that time for being an opaque green stone common in ornamental carvings, but never in jewellery.

Cutting plays an important part in Tanzanite’s colour display. Due to Tanzanite being a trichroic stone (meaning it shows 3 different colours dependant on viewing direction) it shows greenish, violetish or blue. The green element is removed when heating to increase a more desirable violetish blue, and on exceptional occasions a very rare and desirable red flash can occur. This red flash normally only occurs in stones over 5 carats and only in stones with intense colour.

Cutting direction determines the overall ‘face-up’ colour. As with other gems weight loss is profit loss, so cutters consider financial realities when they decide to cut a stone. Larger stones are achievable when cutting direction is used to the benefit of purple tones. So, when an intense blue is the result, it has been the cutters decision to maximise on the rarity and beauty of the stone destined for the high-end market, rather than maximise a more common heavier purple stone. This is why purer blue stones in larger sizes are rarer and in turn the value of these is often greater than an equivalent weight purple stone.

There are 2 distinct markets for Tanzanite. One is for the larger fine quality gems, and an entirely different one for small, calibrated stones. Gem quality comes from just one place on earth - the Merelani area in Tanzania.

 

The rarity of Tanzanite is not only that it comes from one source in the world. The fluctuations on production historically have been due to environmental situations, like flooding in the area, and mining underground becomes more dangerous the deeper it goes. But also, from political instability and changing regulations for export from the country. Control has changed hands repeatedly in its short history of mining, resulting in an inconsistent flow of the gems onto the market.

 

The largest gem found to date weighed 9.27kg in 2020.

 

Other Zoisite varieties have been familiar with gem experts and collectors for far longer than the discovery of Tanzanite. Opaque green is a common medium for ornamental carvings. Pink Zoisite, usually seen as translucent-to-opaque often streaked or mottled with grey or white called Thulite. An attractive new transparent green variety discovered in 1991 also from Merelani location, some are still not quite sure how it should be classified. Some have called them Green Tanzanite (not official). Gemmologists suspect the green colour is caused by chromium, so trade is calling them “Chrome Tanzanite”. Other colours include green-blue, golden, and transparent pink are sometimes marketed as “Fancy-Colour-Tanzanite’s”. These colours are often natural, so the gems are often promoted as untreated.

Mining of tanzanite is expected to end someday, so ownership of this gem one day might well only be achieved through being an heirloom item handed down from generation to generation, or from purchasing at auction.

 

So, if you are lucky enough to own a piece of Tanzanite jewellery your rare and beautiful gemstone is only likely to become rarer with time. If you are looking for a gift for a special someone who has a December birthday, this gemstone would be very thoughtful as well as a timeless and exceptional addition to their jewellery box.