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.
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Tanzanite |
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