#pleochroism

Tanzanite is a gem variety of the mineral epidote. The mineral is normally a calcium-aluminum-silicate mineral, found in low to moderate grade metamorphic rocks. Tanzanite specifically gets its color from substitution of the element vanadium into the mineral; the electrons in vanadium are able to absorb visible light at certain wavelengths, creating the mineral color.

Tanzanite famously has incredible pleochroism, a property of minerals where the way light is absorbed is affected by the polarization. Polarized light is vibrating in only one direction whereas unpolarized light can vibrate in any direction; if you have a pair of sunglasses, they almost certainly are a polarizer as that helps cut down on glare. If light enters a gem quality tanzanite, it will normally give the purple color. However, if the light polarized in one direction as happens if you have sunglasses in front of your eyes, it can show the blue color in the second image. If you turn the sunglasses 90 degrees, it will then shift to the brilliant red color.

Since most light is at least partially polarized by passing through materials or reflecting off surfaces, the pleochroic effect often shows up just in everyday settings, turning the crystal can make it change color. While a number of minerals have this property, Tanzanite’s pleochroism is among the strongest known and very few gemstones show it this intently.

Check out a video example of Tanzanite Pleochroism here at our blog page: https://tmblr.co/Zyv2Js2gx-S8A

-JBB

Image sources: Tanzania Ministry of Minerals: https://www.bbc.com/news/world-africa-53148612

Dr. Rossman: http://minerals.gps.caltech.edu/color_causes/Metal_Ion/Tanzanite_.html