Tourmaline - www.Crystals.eu

Tourmaline

Tourmaline • complex borosilicate cyclosilicate — general: X Y₃ Z₆ (T₆O₁₈)(BO₃)₃ V₃ W Crystal system: Trigonal • Habit: long striated prisms (triangular cross‑section), radial & massive aggregates Mohs: ~7–7.5 • SG: ~3.0–3.3 • Luster: vitreous Colors: all hues, often zoned (bi‑/tricolor; “watermelon”) • Phenomena: strong pleochroism, rare cat’s‑eye Species: Elbaite (Li‑rich), Schorl (Fe‑rich, black), Dravite (Mg‑rich), Liddicoatite (Ca‑rich), Uvite (Mg‑Ca)

Tourmaline — The Encyclopedia of Color in One Mineral Group

Tourmaline is a chemically diverse group of boron‑bearing cyclosilicates prized for its range of colors and strong pleochroism. Crystals are typically long, grooved prisms with triangular cross‑sections. Li‑rich elbaite provides most gem colors (pink “rubellite,” blue “indicolite,” neon Cu‑bearing varieties), while Fe‑rich schorl is black and abundant; Mg‑rich dravite and Ca‑rich liddicoatite round out common gem species.

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What it is
Boro‑cyclosilicate with a ring‑silicate framework. Variable cations (Na, Ca, Li, Mg, Fe, Mn, Al, etc.) occupy multiple sites, producing distinct species and colors.
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Why it captivates
Exceptional color diversity, dramatic pleochroism, frequent color zoning (bi‑/tricolor and “watermelon” slices), and well‑formed crystals.
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Care snapshot
Mohs 7–7.5; no easy cleavage but can be brittle. Avoid ultrasonics/steam and high heat (especially treated stones). Mild soap + water; soft cloth.

Identity & Naming 🔎

Group & structure

Tourmaline is a mineral group (tourmaline supergroup) of 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 material belongs to the elbaite series.

Etymology

The name is linked to the Sinhalese word tōramalli, used historically for mixed colored stones in Sri Lanka. Trade names such as rubellite (red/pink), indicolite (blue), and Paraíba‑type (Cu‑bearing neon blue‑green) describe color/composition within the group.

Color causes: Fe²⁺→ blue/green; Mn²⁺/Mn³⁺→ pink/red; Cr³⁺/V³⁺→ green; Cu²⁺ (with Mn) → vivid “Paraíba” blue‑green; color zoning is common along the c‑axis.

Where It Forms 🧭

Pegmatites

Most gem tourmaline grows in granitic pegmatites from B‑ and Li‑rich late magmatic fluids, with quartz, feldspar, spodumene, lepidolite, and beryl.

Metamorphic rocks

Dravite/uvite occur in calc‑silicate marbles and schists where boron circulates during metamorphism; crystals can be brown to green.

Alluvial deposits

Resistant prisms concentrate in placers. Color‑zoned pebbles and “watermelon” fragments are typical in some districts.

Pegmatites provide the chemistry; long, striated prisms record the flow of color along the c‑axis.

Palette & Pattern Vocabulary 🎨

Palette

  • Green — Fe²⁺/Cr/V.
  • Blue (indicolite) — Fe²⁺; Cu‑bearing shows neon tones.
  • Pink/Red (rubellite) — Mn‑rich.
  • Black (schorl) — Fe‑rich.
  • Bicolor/Watermelon — pink core, green rim (or vice versa).

Tourmaline is typically strongly pleochroic; tone can differ markedly along vs. across the c‑axis.

Pattern words

  • Axial zoning — color changes along the length.
  • Sector zoning — triangular sectors at terminations.
  • “Watermelon” slices — pink core, green rim in cross‑section.
  • Cat’s‑eye — parallel hollow tubes/fibers producing chatoyancy (cabochon).

Observation tip: Use a dichroscope on a green/blue crystal: the two pleochroic directions often appear greenish ↔ bluish or light ↔ dark.

Physical & Optical Details 🧪

Property Typical Range / Note
Chemistry Complex boro‑cyclosilicate: X Y₃ Z₆ (T₆O₁₈)(BO₃)₃ V₃ W; variable cations yield species such as elbaite, schorl, dravite, liddicoatite, uvite.
Crystal system / Group Trigonal • Tourmaline group (supergroup)
Hardness (Mohs) ~7–7.5
Specific gravity ~3.00–3.30 (composition‑dependent)
Refractive index / Optics ~1.614–1.666; birefringence ~0.014–0.032; generally uniaxial (−), sometimes anomalous biaxial; strong pleochroism.
Luster / Transparency Vitreous; transparent to opaque (schorl opaque)
Cleavage / Fracture Indistinct to poor; uneven to subconchoidal fracture; brittle
Electrical properties Pyroelectric & piezoelectric: warmed or rubbed crystals attract dust/ash.
Fluorescence Inert to weak (Mn‑rich pinks sometimes weak red; Cu‑bearing variable)
Treatments Heat to lighten/detone; irradiation to induce/enhance pink; surface coatings rare; note in records where known.
Field ID summary: Long striated prisms with triangular cross‑section, strong pleochroism, RI ~1.62–1.66 (DR 0.014–0.032), SG ~3.1, no easy cleavage. Black tourmaline (schorl) is opaque with high relief.

Under the Loupe 🔬

Growth tubes & “trichites”

Parallel hollow tubes and fine needle‑like inclusions are common; when dense and aligned they can produce chatoyancy in cabochons.

Sector & axial zoning

Triangular sector zoning at the termination and color steps along the c‑axis are diagnostic. Bicolor “watermelon” slices show concentric rims.

Treatment clues

Irradiated pinks may have surface‑concentrated color or fade slightly upon heating; heat‑lightened greens can show subtle residual zoning. Conclusive calls require lab testing.

Look‑Alikes & Imitations 🕵️

Beryl (emerald/aquamarine)

Similar colors; beryl has lower RI (~1.58), SG ~2.7, hexagonal habit, different pleochroism character.

Quartz (amethyst/citrine)

Lower RI (~1.54), SG ~2.65, no strong pleochroism, different crystal habit and inclusions.

Iolite (cordierite)

Strong pleochroism but biaxial with RI ~1.54–1.56 and SG ~2.6; often shows different color trio (blue‑violet↔yellow‑brown).

Corundum (ruby/sapphire)

Higher RI (~1.76), SG ~4.0, different inclusion scene. Tourmaline DR is larger; sapphire often shows doubling less strongly.

Glass & spinel

Glass shows rounded bubbles and single refraction (RI ~1.52); synthetic spinel RI ~1.72 and lacks tourmaline’s pleochroic behavior.

Quick checklist

  • Long striated prism, triangular cross‑section?
  • RI ~1.62–1.66, DR up to ~0.03, strong pleochroism?
  • Color zoning along c‑axis? → Likely tourmaline.

Localities & Uses 📍

Where it occurs

Brazil (Minas Gerais) — classic elbaite of many colors; Afghanistan & Pakistan — vibrant pinks/greens/blues from pegmatites; Madagascar — liddicoatite with striking zoning; Mozambique & Nigeria — Cu‑bearing (Paraíba‑type) and mixed colors; USA — Maine & California pegmatites; Sri Lanka — dravite/varied pebbles.

Common uses

Faceted gems emphasizing pleochroism and zoning, cabochons (especially cat’s‑eye), beads, and polished slices of bicolor/watermelon crystals.

Specimen label template: “Tourmaline (elbaite/dravite/schorl/…) — color — locality — notable zoning/phenomena — treatment (if any).”

Care, Jewelry & Lapidary 🧼💎

Everyday care

  • Mild soap + water; soft cloth dry.
  • Avoid ultrasonics/steam (inclusions, treatments, internal stress).
  • Limit heat and rapid temperature change; store away from harder gems.

Jewelry guidance

  • Orient stones to balance pleochroism vs. saturation (across vs. along c‑axis).
  • Protect thin bicolor slices; avoid open backs where slices are fragile.
  • Cat’s‑eye tourmaline performs best in high domed cabs.

On the wheel

  • Tourmaline is pyroelectric—heat builds static that attracts grit; keep cool, light pressure.
  • Pre‑polish 600→1200→3k; polish with alumina or cerium oxide on tin/leather. Avoid “orange peel” by adjusting lap speed.
  • Check for stress cracks along the length; orient to minimize risk.
Observation tip: Compare color face‑up vs. down the c‑axis in a crystal slice to demonstrate pleochroic tone differences.

Hands‑On Demos 🔍

Pleochroism test

Use a dichroscope on an indicolite or rubellite: two distinct hues/intensities appear and swap with rotation.

Pyroelectric effect

Warm a crystal gently in the hand: small paper ash or lint is attracted to the ends—an accessible physics demo.

Tourmaline is a case study in how chemistry writes color and physics writes optics.

Questions ❓

What is ParaĂ­ba tourmaline?
Cu‑bearing elbaite with vivid neon blue‑green. Originally from Paraíba (Brazil), similar chemistry occurs in parts of Africa; labs typically use “Paraíba‑type” for non‑Brazilian material.

Rubellite vs. pink tourmaline?
“Rubellite” is a trade name for richly saturated red‑to‑deep‑pink elbaite, ideally maintaining color in varied lighting.

Does tourmaline have cleavage?
Cleavage is indistinct; the gem is relatively tough for jewelry but remains brittle—avoid sharp blows.

Why do some stones look darker from one direction?
Strong pleochroism—tone is often deeper down the c‑axis. Cutters orient stones to balance brightness and saturation.

Is irradiation safe?
Yes; gem‑industry irradiation leaves stones non‑radioactive. Treatment should be documented where known.

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