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This month’s birthstone is a relative newcomer to the gemstone world and is found at only one place on earth, its name is tanzanite!
In 1967 a Masai tribes man called Ali Juuyawatu found a few very transparent and rich blue-violet crystals in the hills of Merelani, an area in northern Tanzania near the Kilimanjaro mountain. First it was thought that a new deposit of sapphire was discovered but mineralogical and gemmological research on this material proved that the stones were a variety of the mineral zoisite. This newly discovered beautiful gem was named Tanzanite by Tiffany &Co in honour to its birthplace Tanzania.
The appearance of tanzanite is due to a phenomenon called pleochroism. This is the ability of a gem crystal to show different colours in different viewing directions. These different colours in tanzanite are red-violet, deep blue and yellow green or brownish. The yellow green or brownish colours can be removed by gentle heat treatment at a temperature of 650°C to maximize the blue and violet.
As with all other coloured gemstones, tanzanites with vivid and strongly saturated colours are highly sought after and resemble fine colour sapphires. Every tanzanite is a blend of colours and the exact colour when viewed face-up depends on the way the gem cutter orientates the cut stone.
The tanzanite mining area consist of 4 mining blocks, the largest being block C which is at the centre of the area
What is tanzanite made of?
Tanzanite is a variety of the mineral zoisite and is a Calcium Aluminium Silicate and the element Vanadium is responsible for its blue-violet colour.
How is tanzanite formed?
Tanzanite formed in rocks which were altered by heat and pressure, or by heated fluids from molten rock called magma. When temperature or pressure rises so high that the original minerals can no longer exist, they gradually change into new minerals that are stable under the new conditions. This process can take millions of years. Just imagine that tanzanite is 500 million years old!
A beautiful tanzanite crystal showing the unique intense violet hue (left) and a rare 51.8 ct. tricolour tanzanite crystal.
A phenomenon sometimes encountered in tanzanite is the cat’s eye effect which is caused by its inclusions. Gems like this are cut en cabochon. The gemmologist term for this effect is called chatoyancy, resembling the eye of a cat. This effect is best visible with reflected light.
A rare tanzanite cat’s eye
Care and cleaning
Warm, soapy water is safe, avoid ultrasonic cleaning.
Tanzanite should not be subjected to high temperatures or sudden temperature changes (thermal shock). Also avoid contact with hydrochloric and hydrofluoric acids.
The hardness of tanzanite is 6 to 7, not as hard as sapphire but can still be worn in jewellery with precautions against rough wearing or blows.
Birthstone and anniversary
Along with turquoise, zircon and blue topaz, tanzanite is the birthstone for the month of November. Tanzanite symbolizes the 24th anniversary.