What is Tanzanite?
Tanzania Mined Close to Mount Kilimanjaro
Tanzanite is a rare bluish violet coloured variety of the mineral zoisite. It was first discovered in northern Tanzania in 1967.
After being mined and cleaned stones are often heated very gently. This causes crystals to change colour, sometimes completely. The process takes just thirty minutes at a temperature of no more than 400°C (approx' 734°F).
Unless stated otherwise it's widely accepted that all tanzanite is heated.
The bluish purple colour comes from the presence of vanadium. The scientific name for tanzanite is blue zoisite.
Some tanzanite found at shallow depths naturally exhibit this gem-grade colour but it's incredibly rare. It's caused by geological warming either by metamorphism, wildfires or because of thermal vents within close proximity to where it's found.
Tanzanite that hasn't been heated is far more valuable. Retailers should always know whether a gemstone has been heated and if so, it must be disclosed.
Tanzanite is mined in Merelani Hills on the western slope of the Lelatema Mountains. The area is approximately sixty kilometres from Mount Kilimanjaro.
The deposit is just two kilometres wide by eight kilometres long. Approximately 50,000 people in Merelani earn a living from tanzanite.
There's only a very limited amount of material left available to mine so prices are high.
The exact shade and depth of colour can vary greatly. Tanzanite gemstones with the deepest colour and good saturation command higher prices.
The distinctive purple to violet hue is often the characteristic that stands out the most. Depending on the shade of colour and level of saturation, tanzanite can be confused for blue topaz, iolite, sapphire or aquamarine.
Tanzanite gemstones tend to be faceted and are often set with diamonds.
Tanzanite is pleochroic which means it appears to be different a colour depending on the light and angle from which it's viewed. Under yellow light it always appears to be more violet.
Although correctly known as blue zoisite, the stone was renamed by President and Chairman of the American jeweller Tiffany & Co in 1968. The company introduced tanzanite to the world and said it could only be found in two places on Earth, Tanzania and Tiffany and Co.