Tanzanite, one of December’s birthstones, is a rare blue-to-violet or purple zoisite found only at the foot of Mount Kilimanjaro in Tanzania. Its color can rival that of the finest sapphire. It is a beautiful gem in its own right, however, displaying strong trichroism (or pleochroism), meaning it shows different colors when viewed in different crystal directions.

Tanzanite’s pleochroic colors are red-violet, blue and yellow-green, though most gems are heat treated now, which removes the yellow-green or brownish pleochroic color, maximizing the blue and violet. The tanzanites seen here are also bicolor, which means they have two distinct color zones, in addition to being trichroic!