Properties of Tanzanite
Tanzanite (a variety of zoisite)
Composition Ca2Al2(SiO4)3(OH)
Hardness (Mohs) 6 to 7
Specific Gravity 3.35 (+ 0.10, -0.25)
Refractive Index 1.691–1.700 (0.008–0.013); Doubly refractive, biaxial positive
Crystal System Orthorhombic
Colors Blue, violet, green, brown
Pleochroism Strongly trichroic
Dispersion 0.021
Cleavage Perfect in one direction
Phenomena Cat’s eyes are known. Many tanzanites display a color shift from blue in daylight to violet in incandescent light.
Handling Ultrasonic: Unsafe; never clean tanzanite with ultrasonic cleaners
Steamer: Not safe
The best way to care for tanzanite is to clean it with warm, soapy water.
Acids: Attacked by hydrochloric and hydrofluoric acids
Heat: Fuses under jeweler’s torch; sudden temperature changes may cause cracking.
Enhancements Virtually all tanzanite is heat treated (approx. 500°C), which converts the brown pleochroic color to blue.