Tourmaline — a color encyclopedia in one mineral group
Tourmaline — a chemically diverse group of boron-bearing cyclosilicates, valued for color variety and strong pleochroism. Crystals are typically long, striated prisms with triangular cross-section. Li-rich elbaite provides most gem colors (pink “rubellite,” blue “indicolite,” neon Cu-bearing varieties), Fe-rich schorl is black and abundant; Mg-rich dravite and Ca-rich liddicoatite complement the common gem species.
Identity and name 🔎
Group and structure
Tourmaline is a mineral group (tourmaline supergroup) — boron-bearing cyclosilicates with a complex general formula: X Y₃ Z₆ (T₆O₁₈)(BO₃)₃ V₃ W (X = Na, Ca, K, ☐; Y = Li, Mg, Fe²⁺, Mn²⁺, Al, Cr, V; Z = Al, Mg, Fe³⁺; T = Si±Al,B; V,W = OH,O,F). Most gem-quality material belongs to the elbaite series.
Etymology
The name is linked to the Sinhalese word tōramalli, historically used for multicolored stones in Sri Lanka. Trade names such as rubellite (red/pink), indicolite (blue), and Paraíba type (Cu-bearing neon bluish green) describe color/composition within the group.
Where it forms 🧭
Pegmatites
Most gem-quality tourmaline grows in granite pegmatites from B and Li-rich late magmatic solutions together with quartz, feldspar, spodumene, lepidolite, and beryl.
Metamorphic rocks
Dravite/uvite occur in calc-silicate marbles and shale rocks where boron circulates during metamorphism; crystals can range from brown to green shades.
Alluvial deposits
Resistant prisms concentrate in placers. Some areas feature color zoning pebbles and "watermelon" crystal fragments.
Pegmatites provide chemistry; long, striated prisms record color flow along the c axis.
Palette and pattern dictionary 🎨
Palette
- Green — Fe²⁺/Cr/V.
- Blue (indicolite) — Fe²⁺; Cu-bearing show neon tones.
- Pink/Red (rubellite) — high Mn.
- Black (schorl) — high Fe.
- Bicolor/"watermelon" — pink core, green rim (or vice versa).
Tourmaline is usually strongly pleochroic; tone varies sharply along and across the c axis.
Pattern words
- Axial zoning — color changes along the crystal.
- Sector zoning — triangular sectors near terminations.
- "Watermelon" slices — pink core, green rim in cross-section.
- Cat's eye — parallel hollow tubes/fibers creating chatoyancy effect (in cabochons).
Observation tip: Use a diascopic with a green/blue crystal: two pleochroic directions often show green ↔ blue or light ↔ dark.
Physical and optical details 🧪
| Property | Typical range / note |
|---|---|
| Chemistry | Complex boron-cyclosilicate: X Y₃ Z₆ (T₆O₁₈)(BO₃)₃ V₃ W; variable cations determine species: elbaite, schorl, dravite, liddicoatite, uvite. |
| Crystal system / Group | Trigonal • Tourmaline group (supergroup) |
| Hardness (Mohs) | ~7–7.5 |
| Specific gravity | ~3.00–3.30 (depends on composition) |
| Refractive index / Optics | ~1.614–1.666; double refraction ~0.014–0.032; usually uniaxial (−), sometimes anomalously biaxial; strong pleochroism. |
| Luster / Transparency | Glassy; transparent to opaque (schorl — opaque) |
| Cleavage / Fracture | Unclear to poor; uneven to splintery fracture; brittle |
| Electrical properties | Pyroelectric and piezoelectric: warmed or rubbed crystals attract dust/ash. |
| Fluorescence | Inert to weak (Mn-rich pinks sometimes weakly redden; Cu-bearing vary) |
| Treatments | Heating for lightening/tone correction; irradiation to induce/enhance pink; surface coatings rare; document if known. |
Under the magnifier 🔬
Growth tubes and "trichites"
Common parallel hollow tubes and fine needle-like inclusions; when abundant and aligned, cabochons may show
Sector and axial zoning
Triangular sector zoning at termination and color "steps" along the c axis — diagnostic features. Bicolor "watermelon" sections show concentric rims.
Hints of treatment
Irradiated pinks may have surface-concentrated color or slightly fade when heated; heat-lightened greens may show subtle residual zoning. Laboratory testing is needed for final decisions.
Similar and imitations 🕵️
Beryl (emerald / aquamarine)
Similar colors; beryl RI lower (~1.58), SG ~2.7, hexagonal habit, different pleochroism.
Quartz (amethyst / citrine)
Lower RI (~1.54), SG ~2.65, no strong pleochroism, different habit and inclusions.
Iolite (cordierite)
Strong pleochroism, but biaxial with RI ~1.54–1.56 and SG ~2.6; often shows a different color trio (bluish violet ↔ yellowish brown).
Corundum (ruby / sapphire)
Higher RI (~1.76), SG ~4.0, different inclusions. Tourmaline's DR is higher; sapphire's birefringence often appears weaker.
Glass and spinel
Glass has round bubbles and is singly refractive (RI ~1.52); synthetic spinel RI ~1.72 and lacks tourmaline's characteristic pleochroism.
Quick checklist
- Long ridged prism, triangular cross-section?
- RI ~1.62–1.66, DR up to ~0.03, strong pleochroism?
- Color zoning along the c axis? → Likely tourmaline.
Localities and uses 📍
Where found
Brazil (Minas Gerais) — classic multicolored elbaite; Afghanistan and Pakistan — vivid pink/green/blue from pegmatites; Madagascar — liddicoatite with strong zoning; Mozambique and Nigeria — Cu-bearing (Paraíba type) and mixed colors; USA — Maine and California pegmatites; Sri Lanka — dravite and various schorl.
Most common uses
Faceting emphasizing pleochroism and zoning; cabochons (especially "cat's eyes"), beads, and polished slices from bicolor/watermelon crystals.
Care, jewelry, and grinding 🧼💎
Daily care
- Gentle soap + water; dry with a soft cloth.
- Avoid ultrasound/steam (inlays, treatments, internal stress).
- Limit heating and sudden temperature changes; keep separate from harder stones.
Jewelry guidelines
- Orient stones by matching pleochroism and saturation (across vs. along the c axis).
- Protect thin bicolor sections; avoid open backs where they are fragile.
- The "Cat's Eye" tourmaline looks best in high dome cabochons.
On the wheel (grinding)
- Tourmalines are pyroelectric — static builds up from heat and attracts dust; work coolly, with light pressure.
- Pre-polish 600→1200→3k; polish with aluminum oxide or cerium oxide on tin/leather. Avoid “orange peel” by adjusting the wheel speed.
- Check tension cracks along; orient to reduce risk.
Practical demonstrations 🔍
Pleochroism test
Use a diascopic with indicolite or rubellite: you will see two different colors/intensities that change when rotated.
Pyroelectric effect
Gently warm the crystal in your hand: fine ashes or fluff attract to the ends — a simple physics demonstration.
Tourmaline is a great example of how chemistry “writes” color and physics — optics.
Questions ❓
What is Paraíba tourmaline?
Cu-bearing elbaite with bright neon bluish-green color. Originally found in Paraíba (Brazil), similar chemistry is found in some African locations; non-Brazilian material in labs is often called “Paraíba type.”
Rubellite or pink tourmaline?
“Rubellite” — trade name for rich red–dark pink elbaite, ideally retaining color under various lighting.
Does tourmaline have cleavage?
Cleavage is unclear; the stone is quite suitable for jewelry but remains fragile — avoid sudden impacts.
Why do some stones appear darker from one direction?
Due to strong pleochroism — the tone is often deeper when viewed down the c axis. Cutters orient stones to balance brightness and saturation.
Is irradiation safe?
Yes; irradiation used in the gemstone industry leaves stones non-radioactive. Treatment, if known, should be documented.