Pleochroism in Tanzanite
Tanzanite is noted for its remarkably strong trichroism, appearing alternately blue, violet and burgundy depending on crystal orientation.

Tanzanite is the blue and violet variety of the mineral zoisite (a calcium aluminium hydroxyl sorosilicate) belonging to the epidote group.

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.

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.

Pleochroism in Tanzanite
Gemmy Tanzanite from Merelani Hills (Mererani), Tanzania
Tanzanite’s Interesting Color

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.

Pleochroism in Tanzanite
Image credit: MIM Museum.

Pleochroism in Tanzanite

Pleochroism is a physical property in which the material appears to be different colors when viewed from different crystallographic directions. Tanzanite is pleochroic, meaning the purple, gray, blue and violet tones within the stone vary depending on the angle the stone is viewed from. There might also be a slight color change in incandescent light, when stones may appear to be more violet. Tanzanite is noted for its remarkably strong trichroism, appearing alternately blue, violet and burgundy depending on crystal orientation.

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.

Pleochroism in Tanzanite
Intense purple-pink tanzanite crystal

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Benjamin W said... December 4, 2017 at 8:08 PM

There is single cristal trichroïsme !