Tourmaline
TourmalineJewelryWiki Editorial·5 sources·Reviewed 2026-08-04
An unusually wide specific-gravity range, reflecting how variable the group's chemistry is.
Properties
| Property | Value | Authority |
|---|---|---|
| Refractive index | 1.614–1.666 [1] | — |
| Specific gravity | 2.82–3.90 [1] | — |
| Chemical formula | Na(Al,Li)3Al6(BO3)3Si6O18(OH)4 Tourmaline is a GROUP, not a species. This is elbaite, the lithium-bearing member that most coloured gem tourmaline belongs to. Schorl and dravite are different minerals with different formulas, so a tourmaline of unknown species has no single formula. [5] | — |
| Crystal system | Hexagonal [3] | — |
| Common treatments | Heating can lighten overly dark green material or otherwise shift the colour, and is durable and permanent. Tourmaline is also listed among the durable transparent gems whose surface-reaching fractures can be filled with resins or glasses, though GIA notes this is less prevalent than other treatments. GIA records reports of diffusion treatment in tourmaline but states that the claims have NOT been substantiated — so no diffusion value is published here. [4] | — |
| Birthstone month | October — shares October with opal. [2] | — |
Sources
- International Gem Society — reference charts (Mohs hardness, specific gravity, refractive index) — International Gem Society (IGS reference charts)
- Jewelers of America — Birthstones — Jewelers of America (Jewelers of America, Birthstones)
- Wikipedia — mineral article infoboxes (crystal system) — Wikipedia contributors (Wikipedia mineral infoboxes)
- GIA — An Introduction to Gem Treatments — Gemological Institute of America (GIA, An Introduction to Gem Treatments (Robert Weldon))
- Handbook of Mineralogy — Mineral Data Publishing / Mineralogical Society of America (Handbook of Mineralogy, Mineral Data Publishing)