Tanzanite Gem: Description and Grading
Tanzanite is a purple or blue calcium aluminum hydroxyl silicate. It belongs to the family of Zoisite minerals. It was first discovered in 1967 in Northern Tanzania. The gem is well known for its strong trichroism property – exhibits three different colors (violet, blue and burgundy) when viewed from three different angles. The violet is more prominent under incandescent light and the blue hues are more evident in fluorescent light. In its original state, a Tanzanite gem appears reddish brown in color. It requires to be subjected to 600 degree Celsius of heat to exhibit the brilliant blue-violet color. It is an extremely rare gem and is found in a minable amount only in the foothills of Mt. Kilimanjaro in Tanzania. Some of the early specimens of the gem that were discovered near the surface had a natural blue color and did not need treatment. A Tanzanite is relatively soft compared to other gems and for this reason it is mostly seen only as earrings and necklace ornaments. Gem specimens with bubbles are considered low quality since they tend to increase the size of the Tanzanite.
Colored gemstones have no universally accepted grading system. The Tanzanite Foundation, which is a subsidiary of TanzaniteOne (a major commercial provider of the gem), introduced a grading system for identifying quality and determining the value of Tanzanite. This system divides the gem colors in a variety of hues ranging from violet-blue to blue-violet. Blue is considered the primary hue in the gem and violet is the secondary hue. Purple is an altered hue that lies between blue and red. The deeper the color of the stone, the more valuable it is. The larger specimens are usually deeper in color and it is rare to find an intensely colored Tanzanite that weighs below 5 carats. “Clarity” of the gem is associated with the absence of any natural inclusions or flaws. The gem clarity varies between crystal clear and heavily included. The most valuable Tanzanites are the flawless pieces. The “cut” refers to the gemstone’s proportions, brilliance and finish. The more expensive pieces are the gems that display excellent craftsmanship with cutting. The gem weight is measured in carats. One carat makes 1/5 of a gram. Two identical Tanzanite gems of same sizes can have different carat weights depending on their depths.