Bromoform (tribromomethane, chemical formula CHBr₃) is a dense, colourless to pale yellow organic liquid with a specific gravity of 2.89. For much of the twentieth century it occupied an important place in the gemological laboratory as a heavy liquid — a fluid dense enough to cause lighter minerals to float while heavier ones sink, thereby providing a rapid, non-destructive means of estimating a stone's specific gravity and assisting in species identification. Although largely displaced today by safer alternatives, bromoform remains a reference point in the literature of gemological testing methodology.

Principle of Use

The heavy-liquid method exploits Archimedes' principle in a practical way: a gemstone immersed in a liquid of known density will float if its own specific gravity is lower than that of the liquid, sink if higher, and remain suspended — neither rising nor falling — if the two densities are equal. With bromoform calibrated at SG 2.89, a gemmologist could immediately distinguish, for example, between quartz varieties (SG approximately 2.65, which float) and topaz (SG 3.49–3.57, which sink). Stones hovering near the liquid's surface could be identified as having specific gravities close to 2.89, narrowing the field of candidates considerably.

Bromoform was frequently used alongside other heavy liquids — most notably methylene iodide (diiodomethane, SG approximately 3.32) — to bracket a stone's density between two known values. Together, these liquids formed the backbone of rapid density screening before electronic hydrostatic balances became widely accessible.

Hazards and Regulatory Status

Bromoform is acutely toxic by inhalation, ingestion, and skin absorption. It is classified as a suspected human carcinogen and has been shown to cause liver and kidney damage in animal studies. Vapour pressure at room temperature is sufficient to produce hazardous concentrations in poorly ventilated spaces. Regulatory bodies in the European Union, the United States, and many other jurisdictions have progressively restricted its use, mandating fume-hood operation, chemical-resistant gloves, and strict waste-disposal protocols wherever it remains in use. In several countries, its use outside licensed research facilities is effectively prohibited.

Disposal presents a particular challenge: bromoform cannot be poured to drain and must be collected as halogenated solvent waste for incineration by licensed contractors, adding cost and logistical complexity to any laboratory that retains it.

Modern Alternatives

The principal successor to bromoform and other traditional heavy liquids in gemological practice is sodium polytungstate (SPT) solution, an inorganic salt dissolved in water that can be adjusted across a wide range of densities — from approximately 1.0 to 3.1 — by varying concentration. Sodium polytungstate is considered far less hazardous, is water-soluble, and does not carry the carcinogenic profile of the organohalide liquids. Clerici solution (thallium malonate–formate) reaches higher densities still but introduces thallium toxicity, limiting its appeal. For most routine gemological work, the electronic hydrostatic (specific gravity) balance has superseded liquid immersion methods entirely, offering precision without chemical exposure.

Legacy in Gemological Literature

Despite its withdrawal from routine practice, bromoform appears throughout the foundational gemological literature — in works by Robert Webster, in early editions of the GIA's instructional materials, and in the standard texts of the Gemmological Association of Great Britain. Understanding its role is therefore essential for anyone reading pre-1990s laboratory reports or historical gem-testing records, where notations such as "floats in bromoform" or "sinks in bromoform" served as concise shorthand for a stone's approximate density class. The SG value of 2.89 remains a useful mental benchmark: it falls between the density of most feldspars and that of the corundum group, making it a natural dividing line for a broad range of gem species.