Black tourmaline — the variety schorl — is the most abundant tourmaline and one of the most abundant dark gem materials. It is sold as rough crystals, tumbled stones, beads and bracelets, usually at low prices. That has not stopped substitution: black glass beads, dyed agate, obsidian and even plastic are sold under the name, and a buyer who wants tourmaline specifically has reason to check.
What black tourmaline is
Schorl, an iron-rich member of the tourmaline group (a complex boron silicate). Properties: RI 1.62–1.64 (two readings), SG 3.0–3.25 (heavy for its size, heavier than glass or obsidian), hardness 7–7.5, doubly refractive, opaque black in mass, inert under UV. Crystals are elongated with a rounded-triangular cross-section and deep longitudinal striations along their length — the most recognisable feature of rough tourmaline.
Substitutes and imitations
Black glass. Beads and cabochons. Singly refractive (strain pattern on the polariscope), SG about 2.5, bubbles under strong light, conchoidal fracture; feels lighter than tourmaline.
Obsidian. Natural volcanic glass, SG 2.35–2.45, singly refractive, conchoidal fracture, often slightly translucent brown at thin edges. A legitimate stone; not tourmaline.
Black onyx (dyed agate). Chalcedony, SG 2.60, aggregate reaction on the polariscope, may show banding; dyed.
Hematite and hematine. Metallic lustre, very heavy (SG 5.0+ for hematite; hematine is a manufactured magnetic imitation).
Black spinel, black garnet. Natural, singly refractive, SG 3.6–4.0; sometimes sold interchangeably with tourmaline in beads.
Plastic and resin. Light, warm, soft.
Jet. Fossilised wood, very light (SG 1.3), warm, can be burnt.
How the laboratory identifies it
Weight for size. SG about 3.1 separates tourmaline from glass (2.5), obsidian (2.4), agate (2.6), plastic and jet at once, and from heavier spinel and garnet.
Polariscope. Tourmaline is doubly refractive; a thin edge or a translucent piece blinks. Glass shows strain; agate shows the aggregate reaction; spinel and garnet stay dark. Opaque pieces are tested on any thin, translucent margin.
Crystal form. On rough and on beads cut from rough, the striated prism faces and triangular cross-section are diagnostic.
Refractometer. On any flat polished surface, 1.62–1.64 with two readings.
Magnification. Tourmaline shows fine parallel growth tubes and striations; glass shows bubbles.
Treatments
Black tourmaline is not treated in any routine way; there is no colour to improve. Coating and waxing of tumbled stones is cosmetic.
Frequently asked questions
Is black tourmaline magnetic?
No. Hematine (a manufactured imitation of hematite) is magnetic; schorl is not. Magnetism is a sign the stone is not tourmaline.
Can black tourmaline be translucent?
At thin edges, some schorl transmits a little brownish light. In mass it is opaque.
How does it differ from obsidian?
Obsidian is a natural glass — singly refractive, lighter, with conchoidal fracture and often a glassy lustre. Tourmaline is a crystal — heavier, doubly refractive, striated.
Is a black tourmaline bracelet worth testing?
It is inexpensive, and testing is quick; whether it is worth the fee depends on the price paid and the reason for wanting tourmaline specifically.
Does it generate electricity?
Tourmaline is pyroelectric and piezoelectric — it develops a small charge when heated or squeezed. This is a real physical property but not a practical identification test.