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.

Tanzanite is a trade name that was first used by Tiffany and Company for gem-quality specimens of the mineral zoisite with a blue color. Tiffany could have sold the material under the mineralogical name of “blue zoisite,” but they thought the name “tanzanite” would stimulate customer interest and be easier to market.

The name “tanzanite” was given because the world’s only known tanzanite deposit of commercial importance is in northern Tanzania. The name reflects the gem’s limited geographic origin. The mines are all located in an area of about eight square miles in the Merelani Hills, near the base of Mount Kilimanjaro and the city of Arusha.

Gemmy Tanzanite from Merelani Hills (Mererani), Tanzania

Tanzanite’s Interesting Color

The mineral zoisite naturally occurs in a wide range of colors that include colorless, gray, yellow, brown, pink, green, blue, and violet. The name “tanzanite” is used for a color variety of zoisite that ranges from blue to bluish purple to bluish violet. This type of color-variety name is not unusual. The name “ruby” is used for red to slightly purplish red specimens of the mineral corundum; the name “amethyst” is used for purple specimens of the mineral quartz; and, the name “emerald” is used for green specimens of the mineral beryl. Each of these minerals occurs in a wide range of other colors. The blue color of tanzanite is caused by small amounts of vanadium within the zoisite mineral structure.

Image credit: MIM Museum.

Pleochroism in Tanzanite

Tanzanite is a pleochroic gem. Pleochroism is a physical property in which the material appears to be different colors when viewed from different crystallographic directions. 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.

Intense purple-pink tanzanite crystal