Tanzanite Mineral Guide: Formation Structure and Colour Origins

by Laura Konst

Mineral Overview

Pronunciation: TAN zuh nite

Tanzanite is a blue to violet blue gemstone variety of the mineral zoisite. It is valued for its strong pleochroism, distinctive colour range, and extremely limited geographical occurrence. Unlike many gemstones found across multiple regions, Tanzanite is sourced from a single, small mining area in northern Tanzania, contributing significantly to its rarity.

First identified in 1967, Tanzanite has since become an important mineral both gemmologically and scientifically due to the narrow set of geological conditions required for its formation.

Mineral Classification and Physical Properties

Mineral Group: Silicates
Mineral Category: Sorosilicates
Chemical Formula: Ca₂Al₃(SiO₄)₃(OH)
Crystal System: Orthorhombic
Mohs Hardness: 6.5 to 7
Cleavage: Perfect in one direction
Fracture: Uneven to subconchoidal
Tenacity: Brittle
Refractive Index: 1.691–1.700
Specific Gravity: 3.35–3.55
Birefringence: 0.008–0.013
Pleochroism: Strong (blue, violet and red hues)

Geological Formation

Tanzanite forms under high grade metamorphic conditions within the foothills of the Lelatema Mountains in northern Tanzania. The mineral is found exclusively in the Merelani Hills, an area measuring only a few square kilometres.

Formation occurs when calcium rich rocks are subjected to intense heat and pressure during regional metamorphism. The presence of vanadium within the host rock is essential, as it is responsible for the blue violet colouration that distinguishes Tanzanite from other zoisite varieties. Iron and other trace elements may also influence tonal variation.

Colour and Optical Behaviour

Tanzanite is best known for its strong pleochroism, an optical property that causes the crystal to display different colours when viewed from different angles. Depending on orientation and lighting, Tanzanite may show blue, violet, or reddish tones within the same crystal.

In some specimens this effect is trichroic, meaning three distinct colours are visible along different crystallographic axes. This optical behaviour is a key identifying feature and contributes to both the scientific interest and commercial value of the mineral.

Crystal Growth and Form

Tanzanite typically forms as prismatic crystals, though crystal faces are often incomplete due to geological stress during formation. Natural twin or double crystal formations occur when two crystals grow simultaneously in direct contact. These formations are less common than single crystals and are considered notable from a mineralogical and collecting perspective.

Surface striations and internal zoning may be present, reflecting changes in growth conditions during crystal development.

Stability and Handling Considerations

Despite its moderate hardness, Tanzanite’s perfect cleavage and brittle tenacity mean it is susceptible to damage from impact or pressure. Care should be taken when handling or storing specimens to avoid knocks along cleavage planes.

Brief contact with water is generally considered acceptable, though prolonged exposure may affect surface lustre over time. For preservation, dry dusting is recommended.

Mineral Vault Summary

Tanzanite is a rare zoisite variety formed under highly specific metamorphic conditions and restricted to a single locality. Its strong pleochroism, vanadium driven colouration, and limited geological distribution make it an important and highly collectable mineral. From a scientific perspective, Tanzanite offers valuable insight into trace element influence, optical behaviour, and crystal growth under extreme conditions.

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Love, Laura

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