Tourmaline Group
The tourmaline group including elbaite varieties (rubellite, indicolite, Paraíba), dravite, and chrome tourmaline with properties and identification.
Introduction
Tourmaline is a boron silicate mineral group occurring in more colours than any other
gemstone. Its complex chemistry allows for virtually every colour in the spectrum,
from colourless to black, often in the same crystal.
The name comes from the Sinhalese "turmali," meaning mixed coloured stones.
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
Pink to red tourmaline:
- Colour range: Pink to red to purplish-red
- Chromophore: Manganese (Mn)
- Ideal: Saturated red without brown
- Sources: Brazil, Nigeria, Mozambique, Afghanistan
- Note: Term debated; some require "ruby-like" red
Indicolite
Blue tourmaline:
- Colour range: Light blue to deep blue to blue-green
- Chromophore: Iron (Fe)
- 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
- Discovered: 1989 in Paraíba state, Brazil
- Discoverer: Heitor Dimas Barbosa
- Impact: Created new price category for tourmaline
- Similar material: Found in Nigeria (2001), Mozambique (2005)
Characteristics
- Colour cause: Copper (Cu²⁺), often with manganese
- Unique colours: Electric blue, neon green, violet
- "Glow" effect: Appears luminescent from within
- Typical sizes: Brazilian material usually under 3 carats
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
- 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⁺)
- 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