Gems: Tanzanite


Tanzanite is relatively new to the colored stone galaxy. As the most common story of the tanzanite mining boom goes, in 1967 a Masai tribesman stumbled upon a cluster of highly transparent, intense violet-to-blue crystals weathering out of the earth in Merelani, an area of northern Tanzania.

The instant popularity of this transparent blue-to-violet gem was tied to its vivid color, high clarity, and potential for large cut stones.

  • Mineral: Zoisite
  • Mohs Hardness: 6 to 7
  • Chemistry: Ca2Al3(SiO4)3(OH)
  • Tanzanite is a birthstone for December
  • 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 influenced greatly by its pleochroism, which is the ability of a gemstone to show different colors when viewed in different crystal directions.
  • 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.
  • 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.
  • Sources:,,
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