Tanzanite | The Mineral Blue Zoisite

 

 

tanzanite crystal

 

Tanzanite Facts and Photos


Tanzanite is a variety of blue zoisite that was first discovered in Northern Tanzania in 1967.  It's distinctive bluish lilac colour is mostly achieved by gently heating the stone which causes it to change colour completely or any blue colouration that's already present can be greatly enhanced.  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 colouration is caused by the presence of vanadium in the crystal structure of the mineral zoisite

Some stones that have been found at shallow depths naturally exhibit 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 stone was found.  Tanzanite which has not been heated by man is far more valuable and highly sought after.  Retailers should always disclose whether a stone has been heated but it's something that's usually assumed unless told otherwise.

The area where tanzanite is found lies at the foot of Mount Kilimanjaro and is reported to be two kilometres wide by eight kilometres long.  There's only a limited amount of this mineral left available to mine hence prices are generally very high.  The exact shade and depth of blue colour that's present in tanzanite can vary greatly but stones with the deepest of 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 aquamarie.  Gemstones are mostly faceted and often mounted alongside diamonds.

 

 

two tanzanite crystals one illuminated by white light which has given it a red colouration

 

Photo courtesty of Stan Celestian - Flickr

 

 

 

Tanzanite is pleochroic 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 yellow lighting 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 a distinctive change in colour.  

The scientific name for tanzanite is blue zoisite however it was re named by the President and Chairman of the American jeweller Tiffany & Co shortly before introducing it to the world in 1968.

 

 

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Further Reading

Everything Worth Knowing - Geology.com
The Tanzanite Foundation