Electrical Conductivity Tester
Electrical Conductivity Tester
The instrument that exposed moissanite's semiconductor nature
An electrical conductivity tester — commonly called a moissanite tester — is a handheld gemmological instrument designed to distinguish moissanite (silicon carbide, SiC) from diamond by exploiting a fundamental difference in their electrical properties. Diamond is an electrical insulator in all but the rarest circumstances, whilst moissanite is a semiconductor; applying a small voltage across an unknown stone and measuring whether current passes is therefore a reliable and rapid means of separating the two. These devices became an essential bench tool for gemmologists and jewellers shortly after synthetic moissanite entered the gem market in the late 1990s, when it became clear that existing thermal-conductivity testers were inadequate for the task.
Why Thermal Testers Fail Against Moissanite
Thermal conductivity probes — the standard diamond simulant detectors since the 1970s — identify diamond by its exceptionally high thermal conductivity, which dissipates heat from the probe tip far more rapidly than glass, cubic zirconia, or other common simulants. Moissanite, however, has a thermal conductivity that overlaps with diamond's lower range (approximately 3.5–4.9 W·cm⁻¹·K⁻¹ for moissanite versus roughly 10–26 W·cm⁻¹·K⁻¹ for diamond, depending on type and temperature). In practice, many thermal testers register moissanite as a positive diamond response, or at least produce an ambiguous reading. When Charles & Colvard introduced faceted synthetic moissanite to the trade in 1998, this limitation immediately became a commercial and gemmological problem.
Operating Principle
The electrical conductivity tester applies a low, controlled voltage — typically a few volts — between two contact points placed on the surface of the stone. In diamond, the wide bandgap (approximately 5.47 eV) prevents electron flow under normal conditions, and no current is registered. Moissanite, with a bandgap of approximately 2.36 eV (for the 6H polytype used in most gem-grade material), behaves as a semiconductor and allows measurable current to pass. The instrument's circuitry detects this current and triggers an audible tone, an LED indicator, or both, flagging the stone as moissanite rather than diamond.
The test is non-destructive and takes only a few seconds. Most instruments require the stone to be clean and dry, as surface moisture or residual polishing compounds can introduce spurious conductivity readings.
The Type IIb Exception
A critical caveat governs the use of electrical conductivity testers: type IIb diamonds are also electrically conductive. Type IIb stones contain boron impurities that create p-type semiconducting behaviour, and a conductivity tester will flag them in the same manner as moissanite. Type IIb diamonds are rare — they constitute well under one per cent of all natural diamonds — but they include some of the most celebrated stones in existence, among them the Hope Diamond and the Wittelsbach-Graff. A gemmologist presented with a large, blue or near-colourless stone that triggers a conductivity tester must not conclude automatically that the stone is moissanite; further testing by spectroscopy or a qualified laboratory is required. Reputable instrument manufacturers note this limitation explicitly in their documentation.
Combination Instruments
Recognising that neither thermal nor electrical testing alone is fully conclusive, several manufacturers produce combination units that perform both tests in sequence within a single probe. These instruments first assess thermal conductivity to screen out common simulants such as cubic zirconia and glass, then apply the electrical test to separate moissanite from diamond. A stone that passes the thermal test but triggers the electrical test is identified as moissanite; a stone that passes both tests is consistent with diamond (or, in rare cases, type IIb). Such combination testers have largely superseded single-function thermal probes on the trade bench.
Limitations and Complementary Methods
Electrical conductivity testing addresses one specific identification problem and should not be treated as a comprehensive gemmological analysis. It does not distinguish natural diamond from laboratory-grown diamond, nor does it identify the many other simulants — synthetic rutile, strontium titanate, yttrium aluminium garnet — that a thermal tester would already reject. For definitive identification, particularly of high-value stones, spectroscopic methods (FTIR, UV-Vis, photoluminescence) and grading by an accredited laboratory remain the standard of care. The GIA, for example, provides diamond grading reports that include type classification, which would identify a type IIb stone and preclude misidentification.