Tanzanite: The African gem

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Tanzanite is actually a coloured form of the mineral, zoisite and ranges in colour from an aqua to a deep, royal blue.

Tanzanite uncut crystal

This effect, known as pleochroism, means different colours are yielded from the same stone.

In the rough, tanzanite is often included with brownish-yellow streaks. Heat treating the stone will eliminate these inclusions and this is a popular process to enhance colour. Virtually all tanzanite of a vivid blue hue will have undergone heat treatment; however, heating reduces the stone’s pleochroism.

The most sought-after specimen of the gem is deep blue with a purple shimmer. This shimmer is particularly marked in specimens over ten carats, with smaller crystals being much paler. Large tanzanite crystals are quite rare. Miners utilise heavy picks, iron bars, shovels and jackhammers to extract the gems and these tools can do great damage to rough tanzanite crystals. As tanzanite is soft, it often crumbles during the extraction process.

While it is possible to come across larger crystals, ones weighing two carats or more are quite rare.

Tanzanite was discovered in March 1966 by an Arusha tailor named Sousa at Merelani, (close to Mount Kilimanjaro) Tanzania. Because of its deep blue colour, many suspected this gem was a type of sapphire. On closer inspection and with the use of specialised equipment, experts found that it was actually a coloured variety of zoisite.

Today, tanzainte crystals are derived from several smaller mines in the same location. This is still the only known source of tanzanite. Accordingly, the gem derives its name from its sole country of origin.

New York jeweller Louis Comfort Tiffany, founder of Tiffany and Co., brought world recognition to the gem when he used it in his jewellery pieces. Since then, the stone has become so widely recognised and so well liked that, in 2002, it was added with turquoise and zircon as a traditional birthstone for December.

Despite its popularity in jewellery pieces, tanzanite is not very durable, with a Moh’s hardness rating of 6.5 to 7. For this reason, it should be treated with a degree of care when worn and should not be set in pieces that are likely to be placed at risk of impact, such as rings or bracelets.

The gem can be cut in many different ways, but is most popular as a classical round. Ovals and cushion shapes are also in great demand, with emerald and trillion cuts common also. Tanzanites are scarcely cut into cabochons.

Because of its susceptibility to damage from heat, jewellers are reluctant to resize tanzanite rings as the heat torch required for soldering could shatter the stone completely. Because of this, tanzanite should not be cleaned in ultrasonic machines. The best way to polish tanzanite is with a soft cloth soaked in warm, soapy water and free of acids that might erode the delicate gem.

fact sheet

December – Tanzanite
•Hardness: 6.5 – 7
•Variety of: Zoisite, hydrated calcium aluminum silicate
•Found in: Tanzania

birthstones

December birthstones
•Modern birthstone: Tanzanite
•Traditional birthstone: Tanzanite
•Mystical birthstone: Agate
•Ayurvedic birthstone: Moonstone