The multi-tester is a portable analytical instrument designed to distinguish diamond from moissanite and other common simulants through the combined measurement of thermal conductivity and electrical conductivity. The category emerged in the 1990s in response to the growing commercial availability of moissanite (silicon carbide), which had defeated the previous generation of single-modality thermal-conductivity diamond testers by virtue of moissanite's similarly high thermal conductivity. The combination of thermal and electrical probes in a single instrument provides confident identification of diamond against the most commercially significant simulants, supporting the screening function that is operationally essential at any point in the trade where high-value stones are being acquired or verified.

The diagnostic combination

Multi-testers exploit the complementary diagnostic responses of thermal and electrical probes to confidently distinguish diamond from moissanite. Diamond has exceptionally high thermal conductivity (approximately 2200 watts per metre kelvin for type IIa diamond) but is essentially an electrical insulator at room temperature. Moissanite has thermal conductivity in a similar range to diamond (approximately 480 watts per metre kelvin for the gem-quality 6H polytype, which produces a thermal probe response close to diamond) but is a wide-bandgap semiconductor with measurable electrical conductivity. The combination of high thermal conductivity AND insulating electrical response identifies diamond; high thermal conductivity AND conducting electrical response identifies moissanite; lower thermal conductivity identifies the broader category of common simulants (cubic zirconia, glass, white sapphire, white topaz, white quartz).

The two probes are typically housed in a single integrated probe head, with the operator pressing the probe against the table or crown facet of the stone for measurement. The measurement cycle completes in a few seconds, with the instrument displaying the result through colour-coded indicators or digital readouts referenced to threshold levels.

Operational use

Multi-testers are designed for rapid use in retail, wholesale, and pawnbroking contexts. The principal operational application is the verification of diamond identity in incoming jewellery, particularly in pre-owned and second-hand contexts where the provenance and identity of the stones may be uncertain. The instrument supports rapid screening of mounted and loose stones, with the result providing the necessary confidence for purchase decisions in commercial contexts where the screening level of identification is appropriate.

The instruments are also used in retail customer service contexts, with the rapid confirmation of diamond identity providing reassurance to customers seeking verification of their stones. The visible operation of the instrument and the immediate result delivery supports the broader customer relationship in jewellery retail.

Limitations

Multi-testers have important limitations that the trade should understand and respect. The instruments do not reliably distinguish all synthetic from natural diamonds — particularly newer CVD synthetic diamonds with refined growth processes that produce stones with thermal and electrical properties closely matching natural diamond. The instruments do not identify treated diamonds (irradiated, HPHT-treated, fracture-filled) without supplementary analysis. The instruments do not grade quality or distinguish between specific colour and clarity grades.

For the synthetic diamond identification problem in particular, the multi-tester provides confident identification of diamond as material but does not address the natural-versus-synthetic question that is increasingly central to commercial diamond grading. Stones that pass the multi-tester as diamond but are suspected of being synthetic require submission to specialised testing instruments (the GIA iD100, the De Beers DiamondView, the SSEF Diamond Spotter, and similar dedicated synthetic-diamond screening instruments) or to major laboratory analysis for confident determination.

Manufacturers and pricing

The principal manufacturers of multi-testers include Presidium (Singapore), GemOro (United States), and several smaller manufacturers. Pricing for handheld units suitable for retail use ranges from approximately £100 to £400, with the more sophisticated units offering refinements such as adjustable threshold settings, larger probe ranges, and integrated UV illumination. The instruments have become standard equipment in jewellery retail throughout the developed world.

The relationship to laboratory work

Multi-testers are correctly understood as screening tools rather than as definitive identification or grading instruments. The screening function is essential in trade settings where rapid first-line identification supports operational decisions; the more detailed analysis required for grading, treatment determination, and synthetic identification requires the comprehensive analytical capabilities of major gemmological laboratories. The combination of multi-tester screening at the trade level with laboratory analysis for the stones that warrant the additional detail provides cost-effective verification across the broad range of stones moving through the contemporary trade.

In the trade

For Skyjems and the broader trade, multi-testers are essential operational equipment for the screening of incoming stones. The instruments support rapid confidence in the identification of diamond and the exclusion of moissanite and other simulants, with the laboratory analysis reserved for stones where the additional detail of grading, treatment determination, and synthetic identification is warranted. The instruments are also useful in customer-service contexts, where the rapid demonstration of diamond identity supports the broader retail relationship.

Further reading

• GIA — Gem instruments and analytical methods
• International Gem Society — Diamond testing