Article by Antony Zagoritis (Graduate Gemologist, GIA)
Tanzanite, the blue variety of the mineral Zoisite, is a trichroic gemstone. In layman’s terms this means that it displays 3 different colors on 3 different axes/directions. In the rough state, when Tanzanite is mined, its appearance is largely of a brownish, treacle color. This is due to the dominance of the reddish/brown axis. The photograph below shows some Tanzanite before heating.
The blues and violets that Tanzanite is so well known for do not emerge until the stone is heated. This process is usually done in a gemological oven, at between 400 and 600 degrees centigrade. The heating process removes the brown / burgundy axis, leaving just the blue and violet axes. The heating process serves to render Tanzanite dichroic and permanently changes its original trichroic characteristics.
The photo below shows the same stones after heating.
The Science behind it
In scientific terms, color in Tanzanite is largely caused by the presence of Vanadium. Trace concentrations of vanadium within Tanzanite's crystals structure causes its unique color. It is believed that the Vanadium originates principally from the derived graphite within the deposit. Tanzanite's color largely relies on the Titanium and Vanadium ratio within its crystal structure. The heating of tanzanite results in a valence exchange reaction (redox reaction) Ti 3+ + V 4+ → Ti 4+ + V 3+ which causes an increase in the violet and blue color and renders the titanium colorless.
All Tanzanite is heated. Some pieces are partially heated in the ground and are mined already with some blue but these are generally heated anyway to remove any residual brown. The process is universal to the gemstone and in no way does it affect the value of the stone. Without heating, Tanzanite would be brown and the beautiful blues that are so loved would not be in existence. In fact, legend of the discovery of the stone back in the 1960’s dictates that the Maasai warriors who found it, did so after a large bushfire swept the area. The fire burnt all the foliage and undergrowth back revealing some Tanzanites on the surface and also heated them making them a sparkling blue and suddenly eyecatching. Without the fire, Tanzanite may never have been discovered as the Maasai may not have noticed them in their brownish shades against the bare, burnt earth.
Is Heating Considered an Enhancement in the Trade?
Unlike with other gemstones like Sapphires or Rubies, where heating at extremely high temperatures is used as a way of altering the actual grade of a gemstone, the heating process in no way enhances a Tanzanite’s color saturation levels it simply removes the red/brown axis leaving the blue/violet. A medium grade Tanzanite will appear lightish brown before heating and a medium pale blue/violet after heating. A top grade piece will be a deeply saturated brown before heating and a deep blue/violet after heating. Hence the process is not an enhancement, as it is with some gemstones such as Corundum..
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