Tanzanite - Heat Treated For Aesthetics

Tanzanite 
Ca2Al3(SiO4)3(OH)

 Tanzanite is the gem variety of the mineral Zoisite. Tanzanite’s natural coloring is yellow, brown, green, pale blue not violet blue, which many believe.
 
Today, unfortunately nearly all of the gems being sold as "tanzanite" have a blue color that has been enhanced by heating at relatively low temperatures. The temperature is usually between 360-390 degrees Celsius for 30 minutes. This is accepted in the trade as natural & permanent. The heat treatment transforms its original yellow, brown/green/pale blue (rare) color to the sought after Intense Violet Blue.
 
Note: There are very few natural blue tanzanite crystals left. These were some of the first discovered. Scientist quickly discovered by heating tanzanite it would turn the dull color into a saturated violet-electric blue.

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Image of Natural Tanzanite Below

References:
http://www.tanzaniteamerica.com/Tanzanite-FAQ.php#treated
http://geology.com/gemstones/tanzanite/