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 sorosilicate crystallising in the
orthorhombic system (epidote group). Its key gem variety, tanzanite, is blue-violet
zoisite coloured by vanadium, found only in the Merelani Hills near Arusha, Tanzania,
in a deposit approximately 7 km long and 2 km wide. [1] RI 1.691–1.700
(biaxial positive, birefringence 0.008–0.013), SG ~3.35, hardness 6–7 Mohs. Strong
trichroism (blue, violet, and brown/burgundy in three crystallographic axes) is
diagnostic, separating tanzanite instantly from singly-refractive spinel and from
sapphire (RI 1.762–1.770, SG 4.00). [2] Virtually all gem
tanzanite is heat treated at 500–600 °C to remove the brown axis and develop the
commercial violet-blue; natural blue is a minority. Discovered in 1967 by Maasai
tribesman Ali Juuyawatu and branded "tanzanite" by Tiffany & Co. in 1968, it became
a major market gem within a decade. Its single-source geology and finite deposit
underpin long-term scarcity premiums that have steadily risen.
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. [2]
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
Tanzanite Discovery
Heat Treatment
Virtually all tanzanite is heat-treated:
Treatment Process
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. Read, P. (2008). Gemmology (3rd ed.). Butterworth-Heinemann. ISBN: 978-0-7506-6449-3. DOI: 10.4324/9780080507224.
- ↑ 2. 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.