The GemOro Ultratest is a handheld electronic instrument that combines thermal conductivity and electrical conductivity testing in a single probe, enabling jewellers, appraisers, and gemmologists to distinguish natural diamond and synthetic diamond from moissanite and common simulants quickly and without destructive sampling. It represents a practical response to the commercial introduction of synthetic moissanite in the late 1990s, which rendered single-mode thermal testers — then the industry standard — unreliable for definitive diamond identification.

The Problem It Solves

Thermal conductivity testers, sometimes called diamond probes, became ubiquitous in the jewellery trade from the 1980s onward. They exploit diamond's exceptionally high thermal conductivity — among the highest of any known material — to distinguish it from glass, cubic zirconia, and most other simulants, all of which conduct heat far more slowly. When Charles & Colvard introduced synthetic moissanite (silicon carbide, SiC) as a gem-grade material in 1998, the single-probe tester's limitation became commercially significant: moissanite's thermal conductivity is sufficiently close to diamond's that many thermal-only instruments register a positive "diamond" reading for moissanite. A rapid, reliable screening method that could separate the two materials was therefore needed at the point of sale and in appraisal practice.

Operating Principle

The Ultratest addresses the moissanite problem by adding an electrical conductivity measurement to the thermal test. The two materials differ in a critical electrical property: diamond (type Ia, IIa, or IIb, with the exception of the rare semiconducting type IIb) is effectively an electrical insulator under normal testing conditions, whereas moissanite is a semiconductor and passes a measurable electrical current. The instrument's probe delivers both a thermal pulse and a low-voltage electrical signal to the stone's surface in rapid sequence, then interprets the combined response:

• High thermal conductivity + electrical insulator: consistent with diamond (natural or synthetic, excluding type IIb).
• High thermal conductivity + electrical conductor: consistent with moissanite.
• Low thermal conductivity: consistent with a simulant such as cubic zirconia, glass, or synthetic spinel — regardless of electrical response.

The result is displayed via an LED indicator array or an audible signal, depending on the model variant, allowing a trained user to obtain a preliminary reading in seconds on both loose and mounted stones.

Practical Considerations and Limitations

The Ultratest is a screening instrument, not a definitive gemmological tool, and several caveats govern its use. Type IIb diamonds — natural blue diamonds and some laboratory-grown diamonds doped with boron — are semiconductors and may trigger the electrical conductivity response, potentially causing a false moissanite reading on exceptionally rare natural stones. Users should be aware of this when testing blue or grey diamonds of uncertain origin. Additionally, the probe requires clean, direct contact with the stone's surface; grease, dirt, or metalwork obstructing the culet or table can produce unreliable readings. The instrument is calibrated for polished faceted stones and is less reliable on rough or heavily abraded material.

The Ultratest does not distinguish natural diamond from laboratory-grown diamond — a task that requires spectroscopic analysis (FTIR, UV-Vis, photoluminescence) or referral to a specialist laboratory such as the GIA, IGI, or Gübelin. It also cannot identify coloured gemstones or provide any information about treatments, enhancements, or origin. Its proper role is as a first-pass triage tool: a positive diamond reading warrants further examination; it does not constitute a grading opinion.

Place in Trade Practice

The GemOro Ultratest and instruments of its type are standard equipment at jewellery retail counters, estate-buying operations, and appraisal desks where rapid, non-destructive screening of suspect stones is routinely required. Their value lies precisely in their speed and simplicity: no sample preparation, no laboratory submission, and no specialist training beyond a brief familiarisation with the indicator system. For the working jeweller encountering an unmounted stone of uncertain identity, or assessing a parcel of melee where a single moissanite might be mixed among diamonds, a dual-mode tester provides a defensible preliminary screen that can be completed before any further gemmological work is undertaken.