Tanzanite has had a rapid rise to prominence among jewelers and gem enthusiasts. Although naturally reddish brown, this transparent zoisite variety achieves a stable, beautiful blue to violet color with heat treatments.
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In general, tanzanites showing more blue are valued higher than those showing more violet. Medium dark colors are the ideal. Custom cuts add value. As always, size and clarity have a strong effect on prices. Large clean rough is extremely scare.
Ultra-rare cat’s eye tanzanites are highly prized.
Zoisites, including tanzanite, are Biaxial (+). However, thulite is Biaxial (-). Zoisite varieties have varying optic properties. See "Identifying Characteristics" below.
Biaxial +, Biaxial -
Zoisite is named after Baron Sigmund Zois, who presented the first specimens of the material to Abraham Gottlob Werner, the great mineralogist. Thulite is named after Thule, an ancient name for Norway. Anyolite comes from the Masai anyoli for “green.” Tanzanite is the Tiffany & Co. trade name for blue zoisite, named after the country of origin, Tanzania.
Zoisite occurs in calcareous rocks such as metamorphosed dolomites and calcareous shales subjected to regional metamorphism.
Zoisite may contain fingerprints, healed cracks, and growth tubes. Minerals found included in tanzanite include actinolite, graphite, and staurolite.
Before the discovery of tanzanite, the known varieties of zoisite made little impact on the gem market. Discovered in the shadow of Mt. Kilimanjaro in 1967, tanzanites show both dramatic color change and pleochroism. In natural light, tanzanites can appear an almost sapphire blue. In fluorescent light, they may appear more violet or amethyst-like. Naturally trichroic, these gems can show blue, red-violet, and yellow-green colors when viewed through each of its three crystal axes.
Their most coveted property, however, is their blue to violet color. Although Mother Nature very rarely produces a blue to violet stone in the rough (via the slow heat of the Sun), almost all tanzanites must receive artificial heat treatments to achieve that prized color. Typically, they’re heated to about 500-600º C or 932-1,112º F.
Tanzanites are the most well-known members of the zoisite gem species. Not surprisingly, some vendors will try to associate zoisites of other colors with the tanzanite name. You may encounter green, yellow, and pink zoisites sold as “fancy color tanzanites.” Even though these are rare, beautiful colors, the tanzanite name has more caché with the general public and may command higher prices.
By definition, tanzanites are blue to violet zoisites, whether they receive heat treatment or not. Non-blue zoisites should simply be called zoisites.
Tanzanites will certainly appeal to those interested in truly modern gemstones. Jewelers have even designated this gem a modern option for the December birthstone. However, other gem-quality varieties of zoisite deserve mention, too.
Transparent, gem-quality zoisites occur in many colors and can make beautiful gemstones.
This lapidary rock contains chrome-rich green zoisite, black hornblende, and large, but opaque, ruby. Gem cutters can make cabochons and carvings from this material. This material is also known as ruby-in-zoisite.
There are four “blocks” of tanzanite-producing mines in Tanzania. However, despite claims that “D-Block” tanzanites have superior quality, there’s no difference in tanzanite quality between these mines and no way to distinguish materials from specific mines.
Longido, Tanzania produces deep green crystals, colored by chromium, with ruby crystals.
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For faceting and gem design recommendations for tanzanites and other zoisite varieties, consult this article.
Although tanzanite hardness may range from 6 to 7, its tendency to cleave and brittleness pose hazards for daily wear. Reserve these gems for occasional pieces or use protective settings. Clean them only with a soft brush, mild detergent, and warm water. For more recommendations, consult our gemstone jewelry care guide.