#jewelleryfacts365 226/365 Gem fact

Tanzanite forms as a brownish crystal and is trichroic - which means it shows three colors - brown, blue and violet concurrently. Heating - either occurring underground naturally by metamorphic occurrences or above ground by man in a furnace removes the brown or burgundy color component to produce a stronger purplish-blue color and makes the stone "dichroic" which means it only reflects blue and violet. (source) Some stones found close to the surface in the early days of the discovery (in an area now called D block) were gem-quality blue without the need for heat treatment - probably the result of a wild fire in the area which heated the stones underground.

Tanzanite