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.

Tanzanite’s appearance is significantly shaped by its pleochroism, which is the phenomenon where a gemstone displays different colors when observed from different crystal angles. This characteristic was first documented in scientific literature shortly after the stone’s discovery. In 1969, American Mineralogist described tanzanite’s pleochroic colors as ranging from red-violet, deep blue, to yellow-green. Today, most tanzanites undergo heat treatment, which typically diminishes or eliminates the yellow-green or brownish tones, enhancing the blue and violet hues.

High-quality tanzanite may appear as a violetish blue, similar to that of a fine sapphire, or exhibit a distinct violet shade all its own. Some stones may also take on a more purplish appearance, depending on how the cutter aligns the gem. Both violet and blue pleochroic colors are clearly visible when the stone is gently tilted or rocked. This means that each tanzanite showcases a combination of these pleochroic colors. The final color seen depends on the original rough stone’s color, its size, the cutter’s preferred orientation, and the type of light in which the finished gem is viewed. Cool lighting, like daylight-equivalent fluorescent light, will emphasise the blue tones, while warm lighting, such as incandescent light, brings out the violet to purple shades.

As with other coloured gemstones, tanzanites with vivid, intense colours are highly coveted, whereas lighter, pastel-coloured stones are more common and affordable, offering a more subtle charm.