Tanzanite is a blue-violet variety of the mineral zoisite (a calcium aluminum hydroxyl sorosilicate), caused by small amounts of vanadium. It is a relatively rare gemstone, and is only found in one place in the world: the Mererani Hills of Tanzania.
Tanzanite is a fascinating gemstone known for its captivating color change. It exhibits pleochroism, the ability to exhibit different colors when viewed from different directions. This optical phenomenon arises from the unique arrangement of vanadium ions within its crystal structure. As light passes through the gemstone, it interacts with these vanadium ions, causing different wavelengths of light to be absorbed depending on the orientation of the light and the crystal axes.
The gemstone was given the name 'tanzanite' by Tiffany & Co. after Tanzania, the country in which it was discovered. The scientific name of "blue-violet zoisite" was not thought to be consumer friendly enough by Tiffanys marketing department, who introduced it to the market in 1968. In 2002, the American Gem Trade Association chose tanzanite as a December birthstone, the first change to their birthstone list since 1912.
|Tanzanite trichroism, appearing alternately blue, violet and burgundy depending on crystal orientation.|
According to gemologists , the circumstances that led to the gems formation 585 million years ago were so exceptionally unusual, that the likelihood of finding tanzanite anywhere else on earth is one in a million, making it a thousand times rarer than diamonds. Tanzanite is so rare because it is found and mined in a small area only four kilometers wide and two kilometers long at the foot of Mount Kilimanjaro in the Manyara region of Northern Tanzania.
|Gemmy Tanzanite from Merelani Hills (Mererani), Tanzania|
The mineral zoisite colors are variable but well known as green, blue to violet and pink to reddish in color, Also grey, white or brown. The typical color of tanzanite is blue with a violet tinge. In some lighter colored stones, the color is described as lavender. The color of the tanzanite is the most important factor in determining its price. The blue color of tanzanite is caused by small amounts of vanadium within the zoisite mineral structure.
|Image credit: MIM Museum.|
Pleochroism in Tanzanite
Pleochroism is the optical property of a material that causes it to appear to be different colors when viewed from different directions. This is caused by the way that light is absorbed by the material. In the case of tanzanite, the blue and violet colors are absorbed more strongly in one direction, while the yellow-green color is absorbed more strongly in another direction. This means that when tanzanite is viewed from different directions, the different colors will be absorbed to different degrees, and the resulting color of the gemstone will change.
Some specimens of tanzanite can be a distinct blue when viewed from one direction, and vary from violet to red when viewed from other directions. The group of three photos on this page shows a tanzanite crystal viewed from three different crystallographic directions. Because each of the three directions exhibits a different color, this makes tanzanite a "trichroic" material.
Factors Affecting the Pleochroism of Tanzanite
The pleochroism of tanzanite is affected by several factors, including:
The crystallographic orientation of the gemstone: The colors of tanzanite will be most visible when the gemstone is viewed in a direction that is parallel to the c-axis of the crystal.
The purity of the gemstone: Impurities can affect the pleochroism of tanzanite. For example, iron impurities can cause tanzanite to appear more brownish in color.
The cut of the gemstone: The way that a tanzanite is cut can affect the visibility of its pleochroism. For example, a cabochon-cut tanzanite will show less pleochroism than a faceted tanzanite.
|Intense purple-pink tanzanite crystal|