Zoisite
Zoisite species including tanzanite, thulite, and anyolite with properties, colour change, treatments, and identification.
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.
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
- Geologically unique conditions
- Finite supply concerns affect pricing
- Estimates suggest 20-30 year supply at current rates
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
- 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) |