Tourmaline
Tourmaline is not a single mineral but a group of complex borosilicates sharing a trigonal crystal structure and the general formula XY3Z6(BO3)3Si6O18(OH)4, with composition varying widely among species such as schorl, elbaite, and dravite. It has the widest color range of any mineral group, from jet-black schorl to multicolored elbaite crystals that can shift from pink to green along a single crystal, called watermelon tourmaline. It is also both piezoelectric and pyroelectric, generating an electric charge under pressure or heat.
Tourmaline properties at a glance
| Type | Minerals |
|---|---|
| Chemical formula | NaFe3Al6(BO3)3Si6O18(OH)4 (schorl; composition varies by species) |
| Mohs hardness | 7–7.5 |
| Crystal system | Trigonal |
| Luster | Vitreous |
| Streak | White (colorless) |
| Cleavage | None (indistinct) |
| Fracture | Conchoidal to uneven |
| Specific gravity | 3.0–3.25 |
| Transparency | Transparent to opaque |
| Common colors | Black, Green, Pink, Blue, Multicolored (watermelon) |
| Rarity | common |
Geology & background
Tourmaline crystallizes mainly in granitic pegmatites, where boron-rich residual fluids late in the cooling of a magma concentrate the elements needed to form it, often alongside quartz, feldspar, lepidolite, and spodumene. Gem-quality elbaite comes from pegmatites in Brazil's Minas Gerais, Nigeria, Afghanistan, and California's Pala district, while black schorl, the most common species, occurs widely in granites and metamorphic rocks around the world. Some tourmaline also forms in metamorphic rocks where boron-bearing fluids interact with existing minerals. Collectors especially value elongated prismatic crystals with a rounded triangular cross-section and fine vertical striations running the length of each crystal. Because color zoning is so common, a single tourmaline crystal can show dramatically different colors from base to tip or core to rim, making crystal slices a popular collector and jewelry item.
How to identify Tourmaline
Tourmaline forms elongated, striated prismatic crystals with a distinctive rounded-triangular cross-section, in colors ranging from black and brown to vivid pink, green, and blue, sometimes zoned within a single crystal. Luster is vitreous, streak is white or colorless, and hardness is 7 to 7.5. It has no true cleavage, breaking instead with a conchoidal to uneven fracture. Its triangular cross-section and color zoning usually separate it from beryl and quartz, which lack both features.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Tourmaline is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Tourmaline worth?
| Entry-level specimen | $2 |
|---|---|
| Typical collector specimen | $20 |
| Premium specimen | $150+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Collector specimens
- Education
- Field identification
- Industrial or lapidary use when suitable
Interesting facts
- Tourmaline crystals are piezoelectric and pyroelectric, meaning they generate an electric charge when squeezed or heated, a trait once used to attract ash from pipes.
- Watermelon tourmaline gets its name from crystals zoned pink at the core and green toward the rim, mimicking the fruit's colors in cross-section.
- Black schorl is by far the most common tourmaline species, while gem-quality elbaite crystals represent only a small fraction of what forms.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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