Tourmaline Group
The tourmaline group including elbaite varieties (rubellite, indicolite, Paraíba), dravite, and chrome tourmaline with properties and identification.
Introduction
Tourmaline is a complex boron cyclosilicate group (general formula XY₃Z₆(BO₃)₃Si₆O₁₈(OH)₄)
crystallising in the trigonal system, producing more colour variety than any other gem
species; its multiple substitution sites accommodate lithium, manganese, iron, chromium,
copper, and other elements simultaneously. Key gem species is elbaite (lithium-aluminium).
RI is 1.620–1.640 (uniaxial negative), birefringence 0.018–0.020 (moderately high,
visible doubling in larger stones), SG 3.01–3.11, and hardness 7–7.5 Mohs. [1]
Strong pleochroism (often dark to light of the same hue along and across the c-axis)
governs faceting orientation: dark stones are cut table-perpendicular to the c-axis to
reduce saturation, lighter stones parallel to it to deepen colour. [2]
The most valuable variety is Paraíba tourmaline: copper-bearing (Cu²⁺) elbaite from the
Paraíba state of Brazil, discovered in 1989, displaying unique neon blue-green to violet
colours at concentrations so high they appear luminescent. Fine Brazilian Paraíba can
exceed $50,000 per carat; Nigerian and Mozambican copper tourmaline, chemically similar
but geochemically distinct, trades substantially lower. [3]
Mineralogy
Chemistry and Structure
Tourmaline has a complex formula with multiple substitution sites:
General formula: XY₃Z₆(BO₃)₃Si₆O₁₈(OH)₄
Where X, Y, and Z represent different sites accommodating various elements.
This flexibility produces the remarkable colour range.
Main Species
| Species | Key Elements | Common Colours |
|---|---|---|
| Elbaite | Li, Al | All colours (most gem tourmaline) |
| Dravite | Na, Mg | Brown to yellow to green |
| Schorl | Na, Fe | Black (common, rarely gem) |
| Uvite | Ca, Mg | Green to brown |
| Liddicoatite | Ca, Li, Al | Multicoloured (Madagascar) |
Physical Properties
| Property | Value |
|---|---|
| Crystal system | Trigonal (hexagonal) |
| Hardness | 7–7.5 Mohs |
| Specific gravity | 3.01–3.11 |
| Refractive index | 1.620–1.640 |
| Birefringence | 0.018–0.020 (high) |
| Optic character | Uniaxial negative |
| Pleochroism | Strong (often dark/light) |
Elbaite Varieties
Elbaite is the primary gem tourmaline, named after Elba Island, Italy.
Rubellite
Indicolite
Blue tourmaline:
- Colour range: Light blue to deep blue to blue-green
- Chromophore: Iron (Fe) [2]
- Sources: Brazil, Nigeria, Afghanistan, Namibia
- Note: Pure blue is rare; often greenish
Verdelite
Green tourmaline:
- Colour range: Light to dark green
- Chromophores: Iron, chromium, vanadium
- Most common: Green is the most frequent tourmaline colour
- Sources: Brazil, Africa, Afghanistan, USA
Other Colours
| Variety | Colour | Notes |
|---|---|---|
| Achroite | Colourless | Rare; from Greek 'without colour' |
| Canary | Bright yellow | Malawi, Zambia; manganese coloured |
| Chrome tourmaline | Intense green | Cr coloured; Tanzania, Kenya |
| Bi-colour/tri-colour | Two or more colours | Common; valued for patterns |
| Watermelon | Pink core, green rim | Highly prized slices |
Paraíba Tourmaline
Copper-bearing tourmaline with unique neon colours, among the world's most
valuable coloured stones.
Discovery and History
Characteristics
Origin Comparison
| Origin | Characteristics | Market Position |
|---|---|---|
| Brazil (Paraíba) | Most saturated neon blue | Highest prices |
| Brazil (Rio Grande do Norte) | Similar to Paraíba | High value |
| Nigeria | Good colour; larger sizes | Lower than Brazilian |
| Mozambique | Often lighter; large sizes possible | Lower premium |
Identification
- Must contain copper (Cu), verified by chemical analysis [3]
- Geographic origin requires trace element ratios (Cu/Mn, Ga, Pb)
- Inclusions less diagnostic than for other gems
- Lab certification essential for high-value stones
Paraíba Prices
Chrome Tourmaline
Intense green tourmaline coloured by chromium:
- Chromophore: Chromium (Cr³⁺) [2]
- Colour: Intense, vivid green
- Spectrum: Shows Cr doublet in red
- Sources: Tanzania, Kenya (East Africa)
- Distinction: Richer green than iron-coloured verdelite
- Value: Higher than common green tourmaline
Multicoloured Tourmaline
Tourmaline frequently shows colour zoning:
Bi-colour and Tri-colour
- Colour changes along crystal length
- Common combinations: pink-green, blue-green
- Cut to display multiple colours
- Premium for attractive combinations
Watermelon Tourmaline
- Pink/red core with green outer zone
- Named for resemblance to watermelon slice
- Often cut as slices to show pattern
- Brazilian material especially fine
- Highly collectible with good patterns
Pleochroism Considerations
Characteristic Inclusions
Tourmaline shows distinctive internal features:
- Growth tubes: Parallel to c-axis, often hollow or filled
- Thread-like inclusions: Fine parallel channels
- Colour zoning: Angular, following growth
- Liquid feathers: Healed fracture patterns
- Crystal inclusions: Various minerals depending on source
Identification Summary
Key features for tourmaline identification:
- RI: 1.620–1.640 (useful range)
- Birefringence: 0.018–0.020 (moderately high)
- Strong pleochroism: Dark/light of same hue
- SG: 3.01–3.11
- Spectrum: Variable by colour
- Paraíba: Requires Cu detection for confirmation
References
- ↑ 1. Read, P. (2008). Gemmology (3rd ed.). Butterworth-Heinemann. ISBN: 978-0-7506-6449-3. DOI: 10.4324/9780080507224.
- ↑ 2. Fritsch, E.; Rossman, G. (1987). An Update on Color in Gems. Part 1: Introduction and Colors Caused by Dispersed Metal Ions. Gems & Gemology, 23(3), 126–139. DOI: 10.5741/gems.23.3.126.
- ↑ 3. Abduriyim, A.; Kitawaki, H.; Furuya, M.; Schwarz, D. (2006). "Paraíba"-Type Copper-Bearing Tourmaline from Brazil, Nigeria, and Mozambique: Chemical Fingerprinting by LA-ICP-MS. Gems & Gemology, 42(1), 4–21. DOI: 10.5741/gems.42.1.4.