Presidium Gem Tester
Presidium Gem Tester
The dual-mode thermal-and-electrical-conductivity instrument that separates diamond from moissanite
The Presidium Gem Tester is the family of dual-mode portable instruments — chiefly the Presidium Gem Tester (PGT) and Presidium Multi Tester (PMT) lines — manufactured by Presidium Instruments of Singapore for the screening of diamonds, moissanite, and common diamond simulants. The instruments combine thermal-conductivity testing with electrical-conductivity testing, with the two readings together producing a discrimination that neither test can deliver alone. The dual-mode capability addresses the principal limitation of single-mode thermal testers — the inability to distinguish diamond from moissanite — and has made the Presidium Gem Tester family the standard handheld first-pass instrument in many segments of the trade.
The dual-mode principle
Diamond and moissanite have similarly high thermal conductivity; a single-mode thermal tester reads both as diamond. The two species differ markedly in electrical behaviour: diamond is electrically non-conductive (with the rare exception of Type IIb), while moissanite is electrically conductive. The Presidium Gem Tester applies both tests in sequence — or simultaneously, depending on the model — and produces a combined reading. High thermal plus low electrical conductivity indicates diamond. High thermal plus high electrical conductivity indicates moissanite. Low thermal conductivity indicates a simulant such as cubic zirconia, glass, or synthetic spinel.
The probe tip applies thermal contact through a heated element and electrical contact through a separate electrical pad on the same probe. The instrument processes the dual reading internally and displays a result that distinguishes the three principal categories without requiring the user to interpret the underlying readings.
The PGT and PMT models
The Presidium Gem Tester (PGT) is the higher-capability instrument in the line, with finer sensitivity and additional gem-identification capability beyond the diamond-moissanite-simulant separation. Some PGT models include calibrated readings that suggest gem species across a broader range of materials, though the manufacturer is appropriately cautious about treating these as definitive identification rather than as screening guidance.
The Presidium Multi Tester (PMT) is the lower-cost workhorse model, focused on the diamond-moissanite-simulant separation that drives most counter-side use. It does not extend to broader species identification but performs the principal three-category screening reliably and is the more common instrument in retail and pawn-trade settings.
Use in the trade
The Presidium Gem Tester family is appropriate as a first-pass screening tool in any setting where unidentified or partially-identified stones must be separated quickly. A typical workflow at a retail counter includes the tester applied to the table of each stone in question; the result is read in seconds and the stone is categorised. Stones reading as diamond are forwarded to further verification (loupe, microscope, refractometer where applicable, formal grading where the value justifies it); stones reading as moissanite are flagged for the customer and the trade implications resolved; stones reading as simulants are excluded from the diamond inventory.
The instrument is not a substitute for full gemmological identification of all materials. It is a screening tool: it separates the three principal categories that account for the great majority of counter-side identification questions, but it does not substitute for the broader gemmological battery required for definitive species identification across the wider range of natural and synthetic materials.
Limitations
The Type IIb exception is the principal known limitation. Type IIb diamonds — boron-rich, electrically conductive — read as moissanite on the dual-mode test. The exception is rare but real, and any screening result indicating moissanite on a stone with potential value justifies follow-up examination. Surface coatings on simulants can also produce non-standard readings; the trade has documented occasional cases of coated cubic zirconia or glass producing thermal-conductivity readings that approach diamond range, and the test should be combined with visual examination under magnification rather than treated as the sole identification criterion.