Tanzanite

Tanzanite is a gem-quality variety of zoisite, a sorosilicate belonging to the epidote group. It was first discovered in 1967 near Merelani in the Arusha region of Tanzania by a local gemstone prospector. Its characteristic color, ranging from intense blue to blue-violet, is caused by the presence of vanadium ions (V³⁺) in the crystal structure. Unlike many ancient gemstones, tanzanite has a very recent commercial history: the name itself was coined by Tiffany & Co. in 1968, which recognized the gemological potential of this extraordinary stone.

Tanzanite crystallizes in the orthorhombic crystal system and exhibits a hardness of 6.5 on the Mohs scale, making it relatively fragile compared to diamond, sapphire, and ruby, requiring careful handling in jewelry applications. A fascinating characteristic is its tricolored pleochroism: when observing the crystal from different angles, one can distinguish shades of blue, violet, and red-brown. Many specimens undergo heat treatment to intensify the blue color and reduce unwanted brown tones, a standard practice in the industry. From a geological perspective, tanzanite forms in high-grade metamorphic environments, associated with feldspars, quartz, and other silicate minerals. Known reserves are concentrated almost exclusively in the Merelani deposits, making this gem particularly rare and sought after by collectors.

Chemical formula: Ca₂Al₃(SiO₄)₃(OH). Crystal system: orthorhombic, space group Pnma. Lattice parameters: a ≈ 16.2 Å, b ≈ 5.9 Å, c ≈ 10.0 Å. Mohs hardness: 6.5. Density: 3.35 g/cm³. Refractive index: nα ≈ 1.691, nβ ≈ 1.700, nγ ≈ 1.706 (positive birefringence). Birefringence: Δn ≈ 0.015. Tricolored pleochroism: blue, violet, red-brown (observable with polariscope). Absorption spectrum: characteristic bands around 585 nm and 610 nm due to V³⁺ ions. Luminescence: generally inert to ultraviolet light, occasionally weakly fluorescent. Fracture: conchoidal to irregular. Cleavage: perfect along (010) planes. Thermal stability: stable up to approximately 500 °C; controlled heating at 600–700 °C is commonly employed to improve color by reducing brown components. Typical inclusions: rutile needles, magnetite crystals, rarely fluid inclusions. Primary geographic origin: Merelani deposits, Tanzania (sole significant commercial source). Common treatments: heat treatment, occasionally surface coatings.