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Smithsonian National Museum of Natural History

Zoisite ( var. tanzanite)

Photograph of a tanzanite (NMNH G4876) from the National Gem Collection that displays blue-to-purple pleochroism. In this photo, the crystal is blue but the reflection is purple.
Photo by Chip Clark. Click to zoom.
Catalog Number NMNH G4876-00
Locality Tanzania
Weight 122.7 ct

Obtained in an exchange in 1971.

In 1967, violet blue crystals of the mineral zoisite were discovered in the foothills of Mount Kilimanjaro in Tanzania. These intensely colored crystals were extraordinarily beautiful and propelled zoisite into the ranks of the important gem minerals. In 1968, Tiffany & Co. marketed the new zoisite gem under the name of tanzanite, to honor its single-source availability and its East African birthplace. Since its discovery, tanzanite has steadily increased in popularity. The most valuable tanzanite gemstones are a deep sapphire blue color with highlights of intense violet. Tanzanite exhibits the optical phenomenon of pleochroism, appearing intense blue, violet, or red depending on the direction through which the crystal or gem is viewed. Consequently, the predominant hue of a tanzanite gem depends on the orientation in which it was cut and can therefore resemble a sapphire or even an amethyst in color. All tanzanite gemstones originate from the area where they were initially discovered in Tanzania, where they are mined from metamorphosed limestone. This tanzanite gem weighs 122.74 carats and is the largest faceted tanzanite in the National Gem Collection.

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