Tanzanite Variety of Blue Zoisite Discovered in 1967
Tanzanite is a variety of the mineral blue zoisite that was first discovered in Northern Tanzania in 1967. Its distinctive bluish lilac colour is mostly achieved by gently heating the stone which can cause it to change colour completely. Heat is also used to enhance any blue colour that's already present. The process takes just thirty minutes at a temperature of no more than 400 °C (approx' 734 °F) and unless otherwise stated, it's widely accepted that all tanzanite is heated.
The blue colour in tanzanite is caused by the presence of vanadium in the crystal structure of the mineral zoisite.
Some tanzanite that has been found at shallow depths naturally exhibits this gem-grade colour but it is relatively rare. It's caused by natural geological warming either by metamorphism, wildfires or because of thermal vents within close proximity of where the mineral was found. Tanzanite which has not been heated by man is far more valuable and highly sought after. Retailers should always know whether a stone has been heated and the information should always be disclosed.
The area in Tanzania where tanzanite is found lies at the foot of Mount Kilimanjaro. According to reports it's just two kilometres wide by eight kilometres long. There's only a limited amount of this sought after mineral left available to mine hence prices tend to be very high.
The exact shade and depth of blue colour that's present in tanzanite can vary greatly but stones with the deepest colour and good saturation command higher prices. Its distinctive purple to violet hue is often the characteristic which stands out the most. Depending on the shade of colour and level of saturation, tanzanite can be confused for blue topaz, sapphire or aquamarine. Gemstones are mostly faceted and often mounted alongside diamonds.
Photo Courtesy of Stan Celestian - Flickr (clickable image)
Tanzanite is a pleochroic mineral which means it appears to be different a colour depending on the type of light that's present and the angle from which it's viewed. Under a yellow light it always appears to be more violet. The crystal on the right in the photo above has been illuminated by white light from underneath which has caused the distinctive change in colour.
The scientific name for tanzanite is blue zoisite however it was re named by President and Chairman of the American jeweller Tiffany & Co shortly before they introduced it to the world in 1968.
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