THERE鈥橲 always something new out of Africa, complained the Roman Pliny the Elder, as did Aristotle before him. They didn鈥檛 approve of novelty. But even they might have been impressed with the latest geological sensation from the continent: the world鈥檚 biggest lump of tanzanite, found 270 metres below ground in a mine near Mount Kilimanjaro in Tanzania last month.
Tanzanite is the newest of the precious stones, and one of the most sought-after because of its rich lavender-blue-azure colour; blue gemstones are rare. Tanzanite is considered fifth in line after the 鈥渂ig four鈥�: diamond, emerald, ruby and sapphire. Discovered in 1967 by Masai herdsmen, its only known source is an area of about 13 square kilometres in the Usambara mountains near Arusha in the north of Tanzania 鈥� hence the name. Tanzanite is rarer than diamond, so you can imagine the excitement of the Tanzanian miners who uncovered the latest monster nugget. It is the size of a brick and weighs more than 3 kilograms 鈥� that鈥檚 enough for about 30,000 engagement rings 鈥� and could be worth up to $15 million.
What gives tanzanite its blue? The mineral is calcium aluminium silicate hydroxide, a variety of zoisite, a fairly common rock-forming mineral. In its natural state tanzanite is a glassy brown or battleship grey, but it turns azure-purple when heated to 300 to 400 掳C. The Masai discovered examples scattered over the ground that had been turned blue by heat from bush fires started by lightning. Miners often bring out the colour themselves by baking the crystals in a furnace 鈥� rumour has it by wrapping them in dough beforehand to protect them. The blue comes from trace amounts of the metal vanadium which when heated causes the configuration of electrons within tanzanite鈥檚 crystalline structure to shift.
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鈥淗ow did this precious stone end up in a small corner of Tanzania?鈥�
So how did this precious stone end up in a corner of Tanzania? Superheated water from deep within the Earth鈥檚 crust dissolved minerals and metals, and carried them towards the surface through fissures in a bedrock of gneiss. As the solution neared the surface it cooled, and mixtures of minerals crystallised out. The rarity of tanzanite has fuelled a modern gold rush, with local miners scrambling for precious licences. And of course, watch out for fake versions.
What will happen to the giant tanzanite crystal? The mining company, TanzaniteOne, says the rock will go on display before being cut up.
The firm has named the lump the Mawenzi, after the second highest of Kilimanjaro鈥檚 three peaks. They didn鈥檛 want to name it after the highest peak, Uhuru, on the off-chance that someone might find a larger piece.
