Zoisite

Zoisite species including tanzanite, thulite, and anyolite with properties, colour change, treatments, and identification.

By Fabian Moor Last updated
tanzanite species/zoisite thulite anyolite

Introduction

Zoisite (Ca₂Al₃(SiO₄)₃(OH)) is a calcium aluminium silicate that produces
several gem varieties. Tanzanite – the blue-violet variety found only in
Tanzania – is by far the most important, having become one of the world's
most popular coloured gemstones since its discovery in 1967.

Mineralogy

Crystal System and Structure

  • Crystal system: Orthorhombic
  • Chemical formula: Ca₂Al₃(SiO₄)₃(OH)
  • Habit: Prismatic crystals, often striated
  • Cleavage: Perfect in one direction
  • Related: Epidote group member

Physical Properties

Property Value
Hardness 6–7 Mohs
Specific gravity 3.10–3.38
Refractive index 1.691–1.700
Birefringence 0.008–0.013
Optic character Biaxial positive
Pleochroism Strong (trichroic in tanzanite)
Lustre Vitreous

Tanzanite

Tanzanite is the violet-blue variety of zoisite, coloured by vanadium. [1]
Found in only one location on Earth, it has become a major gemstone
since its 1967 discovery.

Colour and Pleochroism

Tanzanite shows strong trichroism:

  • Axis 1: Blue
  • Axis 2: Purple/violet
  • Axis 3: Brown/burgundy (often heated away)

The face-up colour depends on cutting orientation. Most stones are
heat-treated to remove brown and enhance violet-blue.

Colour Grading

Tanzanite colour is graded by:

  • Hue: Blue to violet (pure blue most valuable)
  • Saturation: Vivid preferred
  • Tone: Medium to medium-dark
  • "AAA" quality: Trade term for finest blue

The Single Source

Tanzanite occurs only in a small area of Tanzania:

  • Merelani Hills, near Mt. Kilimanjaro
  • Deposit approximately 7 km long, 2 km wide [2]
  • Geologically unique conditions
  • Finite supply concerns affect pricing
  • Estimates suggest 20–30 year supply at current rates [CITATION NEEDED]

Tanzanite Discovery

Heat Treatment

Virtually all tanzanite is heat-treated:

Treatment Process

  • Natural tanzanite often brownish or burgundy
  • Heating to 500–600°C removes brown [1]
  • Enhances violet-blue colour
  • Treatment is permanent and stable
  • Assumed unless stated otherwise

Detection

Detecting heat treatment in tanzanite is difficult:

  • No reliable visual indicators
  • Some natural blue exists (minority)
  • Laboratory techniques can sometimes detect
  • Market assumes all is heated

Other Zoisite Varieties

Thulite

Pink to red opaque zoisite:

  • Colour cause: Manganese
  • Character: Opaque to translucent
  • Uses: Cabochons, carvings, beads
  • Sources: Norway (type locality), Austria, USA

Anyolite (Ruby-in-Zoisite)

Green zoisite with ruby inclusions:

  • Character: Green matrix with red ruby crystals
  • Black inclusions: Hornblende
  • Source: Tanzania
  • Uses: Carvings, cabochons, decorative objects
  • Note: Ruby usually not gem quality

Chrome Zoisite

  • Rare green transparent zoisite
  • Coloured by chromium
  • Very limited availability
  • Collector interest

Durability Concerns

Identification Summary

Key features for tanzanite identification:

  • RI: 1.691–1.700 (useful range)
  • SG: 3.35
  • Birefringence: 0.008–0.013
  • Pleochroism: Strong trichroism (diagnostic)
  • Single source: Tanzania only
  • Spectrum: Vanadium-related features

Distinguishing Similar Gems

Gem Key Distinction from Tanzanite
Sapphire Higher RI (1.76–1.77); higher SG (4.0)
Iolite Different pleochroic colours; lower RI
Spinel Spinel is SR (no pleochroism)
Amethyst Lower RI (1.54–1.55); lower SG (2.65)

References

  1. ↑ 1. Pluthametwisute, S.; Wanthanachaisaeng, B.; Saiyasombat, C.; Sutthirat, C. (2020). Cause of Color Modification in Tanzanite after Heat Treatment. Molecules, 25(16), 3743. DOI: 10.3390/molecules25163743.
  2. ↑ 2. Read, P. (2008). Gemmology (3rd ed.). Butterworth-Heinemann. ISBN: 978-0-7506-6449-3. DOI: 10.4324/9780080507224.

Related Topics

Physical PropertiesOptical Properties