Tanzanite is a gem noted for its bright colour, its high degree of clarity, and its almost unique source. Tanzanite is an unusual gem because it is found near Arusha, Tanzania, and really not in any commercial quantities anywhere else. The gem was popularized in 1967 by Tiffany & Co (tiffany.com).
One of the fascinating features of Tanzanite is its pleochroism. Pleochroism is a directional difference in light absorption, leading to differences in its appearance when examined from different directions. The finest tanzanite is a rich blue when examined face-up, but will show violet colours when examined from different directions.
Tanzanite really shows strong colour differences when viewed from different angles. This is shown to great effect when this same tanzanite is viewed in different positions between polarizing filters in a gemmological polariscope. Above and below you can see the distinct directional differences, face-up and between filters.