Tanzanite is the violet blue to blue violet variety of the mineral zoisite. It is mined commercially only in one area of the world: the Merelani Hills of Tanzania, which is where it gets its name.
In 1969, American Mineralogist described the gem's pleochroic colors as `red-violet, deep blue, and yellow green.' Today, most gems are heat treated, which removes or reduces the yellow green or brownish pleochroic color, maximizing the blue and violet.
Top-quality tanzanite can be violetish blue --similar to a fine sapphire color --or a unique, predominately violet hue all its own. Some stones might also appear more purplish depending on how the cutter chooses to orient the fashioned gem. Both the violet and blue pleochroic colors are readily visible in a fashioned stone when it is gently rocked and tilted. This means that every tanzanite is a blend of these pleochroic colors. The exact face-up color depends on the inherent color of the original rough, its size, the pleochroic colors the cutter favors when they orient the fashioned stone, and the light the finished gem is viewed under. Cool lighting --like daylight equivalent fluorescent --will emphasize tanzanite's blue, while warm lighting --like incandescent --will make it appear more violet-to-purple.
Just like other colored gemstones, vivid strongly-colored tanzanites are highly sought after. Lighter toned pastel hues are more plentiful and affordable than vivid colors and have a subtle appeal of their own.
The above passage is extracted from www.gia.edu.