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) |