Why Thermal Testers Fail on Moissanite—and What US Jewelers Use Instead

Jewelers across the U.S. still rely on handheld thermal testers every day. But those same testers routinely mistake moissanite for diamond. That mismatch has real consequences — from mispriced stock to unhappy customers. This article explains, in plain terms, why thermal testers fail on moissanite, what physical properties make the two stones similar, and which practical tests and tools jewelers use today to tell them apart reliably.

Why thermal testers were designed — and why they fail on moissanite

Thermal testers (often called “diamond testers”) measure how fast heat moves away from a small heated probe when it touches a stone. Diamond conducts heat much faster than most common simulants like glass or cubic zirconia. That made thermal testing a quick and affordable way to screen rings on the bench.

The problem: moissanite (silicon carbide, SiC) also conducts heat very efficiently. Its thermal conductivity is close enough to diamond that a probe calibrated only for heat flow will record a “diamond” reading. In other words, both stones behave the same way thermally, so a test that looks only at heat transfer cannot distinguish them.

What other physical clues help identify moissanite

• Optical properties — refractive index and dispersion. Diamond’s refractive index (RI) is about 2.42. Moissanite’s RI is higher (commonly cited around 2.65–2.69). That higher RI and greater dispersion (moissanite’s dispersion ≈ 0.104 vs diamond ≈ 0.044) produce stronger “fire” — colored flashes — in moissanite. On a well-cut stone this can be a visible difference under a loupe or microscope.
• Birefringence (double refraction). Moissanite is doubly refractive; diamond is singly refractive. Under a loupe or low-power microscope you may see doubled facet junctions or doubled facet reflection lines in moissanite, especially on pavilion facets. This is one of the most reliable bench clues when you can inspect the stone closely.
• Specific gravity (density). Diamond’s specific gravity is about 3.52. Moissanite’s is about 3.21. The difference is measurable with a hydrostatic balance or density kit for loose stones. It’s not practical for set stones without removing them, but it’s precise when you can use it.
• Electrical behavior. Moissanite is a semiconductor and often shows electrical conductivity; most diamonds are electrical insulators. That difference is precisely what modern “dual” testers use to separate moissanite from diamond.

How modern testers and workflows solve the problem

Once moissanite became common in the market, instrument makers added electrical testing to thermal probes. These “dual testers” or moissanite-capable testers measure both thermal and electrical responses. The logic is simple: if a stone reads high on heat transfer but shows electrical conductivity, it’s likely moissanite. If it reads high thermally and shows no conductivity, it’s likely diamond.

That approach works in most retail and bench situations, but it isn’t foolproof. Rare diamonds with boron impurities (blue diamonds) or certain lab-grown or treated stones can show unusual electrical responses. That’s why good shops use a combination of tests rather than a single instrument.

Recommended practical workflow for U.S. jewelers

• Step 1 — Loupe and microscope inspection (10x–30x). Look for doubled facet reflections, facet junction behavior, and unusually strong colored flashes. Moissanite often shows doubled lines or a telltale “double window” effect on certain facet angles.
• Step 2 — Thermal + electrical (dual) tester. Use a device that reports both parameters. If heat is high and electrical conductivity appears, treat the stone as moissanite until proven otherwise.
• Step 3 — Polariscope or crossed polarizers. Moissanite is anisotropic and will show characteristic strain or extinction behavior; diamond will not. This is a quick secondary check on mounted stones with careful technique.
• Step 4 — Specific gravity or refractometer (loose stones). If the stone is loose, measure SG or RI. Moissanite’s RI and SG differ from diamond enough to be decisive.
• Step 5 — Laboratory confirmation when needed. If a sales dispute, certification, or insurance requirement calls for certainty, send the piece to a gemological lab. Raman spectroscopy or advanced spectroscopic testing will positively identify moissanite vs diamond.

Tools U.S. jewelers commonly use

• 10× loupe and binocular microscope — essential for spotting double refraction and excessive dispersion.
• Dual thermal/electrical tester — the everyday handheld tool that replaces old thermal-only testers.
• Polariscope and darkfield light — useful for anisotropy checks and inclusion patterns.
• Refractometer and hydrostatic balance — for loose stones to measure RI and specific gravity precisely.
• UV lamp — moissanite and diamond can show different fluorescence patterns; this is a supporting test, not definitive.
• Raman spectrometer — used by labs and high-volume dealers for conclusive identification.

Practical notes and caveats

• Train staff to use a combination of tests. Relying on a single thermal probe invites mistakes. Explain why: thermal alone cannot separate two materials with similar heat conductivity.
• Keep a known moissanite and diamond reference on the bench. A physical comparison accelerates learning and reduces false reads.
• When in doubt — disclose and escalate. If a customer questions a stone’s identity, disclose the tests you ran and offer lab confirmation. That protects both reputation and legal risk.

Bottom line

Thermal testers fail on moissanite because moissanite and diamond move heat similarly. The practical response for U.S. jewelers is not to abandon electronic testing but to upgrade to dual thermal/electrical testers and pair those devices with visual inspection, polarization, and lab-grade methods when needed. That layered approach gives fast, reliable answers at the bench and a clear path to definitive identification when stakes are high.