Tanzanite

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Tanzanite is the blue-violet variety of the mineral zoisite, distinguished by a pronounced trichroism that shifts through blue, violet, and burgundy as the stone is turned—one of the most visually mesmerizing effects in the gem world. It is found only in a small mining area near Mount Kilimanjaro in northern Tanzania, was discovered in 1967, and was named and popularized by Tiffany & Co. Estimated to be a thousand times rarer than diamond, tanzanite draws lasting demand, though its single source makes long-term supply uncertain. A Mohs hardness of 6.5 to 7 calls for protective settings, and the stone serves as a December birthstone.

Physical & Optical Properties

Category Semi-Precious Gems

Mohs Hardness 6.5 – 7.0

Specific Gravity 3.35

Refractive Index 1.685 – 1.707

Crystal System Orthorhombic

Cleavage Perfect

Fracture Uneven

Color various

Birefringence 0.006

Dispersion 0.03

Pleochroism Violet/purple stones - strong trichroism: strong blue - purple - greenish (to brownish) yellow; Blue stones - strong: strong blue - purple - greenish-yellow

Mohs Hardness

Mohs hardness of Tanzanite shown against the ten reference minerals, from talc (1) to diamond (10).

Mohs	Reference mineral	Tanzanite
1	Talc
2	Gypsum
3	Calcite
4	Fluorite
5	Apatite
6	Orthoclase
7	Quartz	●
8	Topaz
9	Corundum
10	Diamond

Crystal System

Value & Market

Price Tier Moderate ($50-500/ct)

Where to Buy Tanzanite

Blue Nile Fine jewelry retailer

James Allen Gemstone jewelry

Brilliant Earth Ethically sourced gems

Amazon Wide selection

Links may earn a commission at no extra cost to you.

Cultural Significance

Birthstone Month 12 (modern)

Treatments

6.5 to 7Walter Schumann, Gemstones of the world (2001) More from other references 3.35Walter Schumann, Gemstones of the world (2001) More from other references BrittleMichael O?Donoghue, Gems, Sixth Edition (2006) PerfectWalter Schumann, Gemstones of the world (2001) UnevenWalter Schumann, Gemstones of the world (2001) 1.685 to 1.707Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) More from other references Biaxial/+Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) More from other references 0.006 to 0.013Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) More from other references Violet/purple stones - strong trichroism: strong blue - purple - greenish (to brownish) yellow; Blue stones - strong: strong blue - purple - greenish-yellowHerve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) More from other references 0.030Walter Schumann, Gemstones of the world (2001) YesUlrich Henn and Claudio C. Milisenda, Gemmological Tables (2004) Sapphire blue, amethyst, violetWalter Schumann, Gemstones of the world (2001) More from other references Blue, (heat treated), V4+ in octahedral coordination; with V3+ in octahedral coordination. Brown-violet, V3+ in octahedral coordination (treatment turns blue). Green, Cr3+ in octahedral coordination.W. William Hanneman, Pragmatic Spectroscopy For Gemologists (2011) TransparentUlrich Henn and Claudio C. Milisenda, Gemmological Tables (2004) More from other references InertHerve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) Rare bluishHerve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) OrthorhombicHerve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) Multifaced prisms, mostly striatedWalter Schumann, Gemstones of the world (2001) Tanzanite information at mindat.org Tanzania  Malisa (2003) Malisa (2003) Malisa (2003) Malisa (2003) Malisa (2003) Malisa (2003)