Treatment of Tanzanite Gems

Tanzanite is the gem name for the silicate mineral Zoisite.  Although this fairly common mineral has long been known to geologitsts, the gem form of this   mineral was only discovered in 1967. While there are a few other deposits, virtually all the gem quality material used in the trade is recovered from a single large geologic zone about 5 miles long, located in the Merilani Hills of Tanzania, Africa. That of course, is how the gem acquired the name of tanzanite. Few people know that the gem tanzanite actually comes in a variety of colors, including yellows, greens and of course the best known color of blue shading to purplish blue.   This best known coloration resembles the finest Sapphire in many ways, and is the source of its great popularity.  Some of the green shades of this gem are actually colored by the presence of the element chromium and resemble a very fine emerald.  Stones of this emerald green hue are extremely rare, and the color is not achieved through any form of treatment.

Few people also know that the color of the well-known beautiful blue violet of Tanzanite is not a natural color for most of the material that is mined.   The vast majority of all tanzanite as it is mined comes out of the ground displaying a brownish color. This material actually shows a strong violet red / deep blue / yellow green trichroic coloration, meaning that different colors are displayed when the stone is viewed in different directions. The mixed combination of these different colors appear under normal viewing as a brownish to brownish violet kind of coloration.  Some of the best natural stones actually have a coloration similar to that of root beer before treatment.   A gentle heating for about two hours at a temperature of 370°C will convert the brownish natural material to the desired violet blue coloration. Application of this heat for too long a time period will change fine colored stones to a light blue color, that is less desirable. In heating Tanzanite, it is best to raise and lower the temperature of the material very slowly as the gem is somewhat fragile toward heat, and rapid changes may cause fractures.  It is common practice to cut and polish the rough stones in their untreated form, and remove all the flaws and imperfections as a part of the cutting process.  That way only the cleanest, most imperfection free parts of the stone are the  ones that are then treated to produce those beautiful sapphire blue color gems.  This helps to reduce the potential for any unintended cracking or other damage which may occur during heating process. Temperatures more than about 400° centigrade are usually unnecessary, and on heating to temperatures over 900° centigrade will remove water from the crystal, cause it to become cloudy and destroy the stone.