As far as precious gemstones are concerned, tanzanite is a relatively recent discovery. A form of the mineral Zoisite, tanzanite was discovered in 1967 by Manuel D’Souza in the Manyara region near Mount Kilimanjaro, Tanzania. Whilst only discovered in the 20th century, geologists suggest that the gemstone was likely formed over 500 million years ago through immense heat combined with tectonic activity around Mount Kilimanjaro during the Ediacaran Period.
Originally referred to as ‘Blue Zoisite’, Tiffany & Co. launched a marketing campaign that rebranded the new gemstone as ‘Tanzanite’ to reference the exotic geographical location of discovery. Since then, various international jewellery houses including Bulgari, Buccellati, and Cartier among others have incorporated this alluring stone into their designs. Some notable examples include the Chopard ‘Belle’ necklace inspired by Disney’s Beauty and the Beast, Harry Winston’s 1991 convertible ‘Peterson’ brooch in platinum, and the intricate ‘Queen of Kilimanjaro’ tiara centring a 242 carat faceted tanzanite within a contrasting surround of diamonds and tsavorite garnets.
Perhaps the most remarkable feature of tanzanite is the intense and unique colour. Often mined rough as a reddish-brown coloured stone, tanzanite is heated to remove these tones and reveal dominant violet hues. The process is much gentler than typical gemstone heating, with temperatures of around 500 – 800 degrees Celsius for up to an hour. This treatment is considered stable and permanent.
Intriguingly, tanzanites display an optical phenomenon called pleochroism. Depending upon crystallographic orientation, the mineral can absorb different transmitted light wavelengths. This means that the stone displays a change of colour when observed from different directions or when viewed through plane polarised light. As tanzanite is trichroic, it is possible to observe three different colours when viewed from varying angles.
Colours visible in tanzanite include violet, blue, indigo, burgundy, purple, red, brown, green, yellow, and cyan. Due to these colour variations, it requires a skilled lapidarist to cut a tanzanite ensuring the most desirable colour is reflected through the table of the stone.
Along with high clarity and exceptional colour display, scarcity and rarity remain a drawcard for gem collectors. Mines are close to depletion, with geologists speculating that supply could run out within the next 20 years. In terms of value, a watershed moment will be when the mine is entirely depleted, and tanzanite is considered a highly coveted ‘heritage’ stone.
The October 2022 Fine Jewels and Timepieces auction features several pieces of collectable tanzanite jewellery.
BETHANY MCGOUGAN / Head of Fine Jewels & Timepieces