When a standard diamond tester touches moissanite, it reads diamond. This surprises buyers, confuses jewelers, and occasionally causes concern. But the reason is simple physics — and understanding it gives you complete confidence in your stone and the knowledge to handle any testing situation. This is the full scientific explanation.
Two Types of Diamond Testers — And Why They Give Different Results
There are two fundamentally different categories of diamond testing devices, and they test completely different physical properties:
Thermal testers measure how quickly a stone conducts heat away from a heated probe tip. Diamond conducts heat exceptionally well — faster than almost any other material. When a thermal tester probe touches diamond, heat dissipates rapidly, and the device reads this as "diamond."
Electrical testers measure whether a stone conducts electricity. Diamond does not conduct electricity — it is an electrical insulator. Electrical testers were designed to distinguish diamond from moissanite specifically, because moissanite is a semiconductor.
Why Moissanite Passes the Thermal Test
Moissanite (silicon carbide, SiC) has exceptionally high thermal conductivity — approximately 490 W/m·K, compared to diamond's 900–2,320 W/m·K depending on type. This is dramatically higher than any other gemstone: cubic zirconia conducts heat at approximately 2 W/m·K, sapphire at approximately 35 W/m·K.
Because moissanite's thermal conductivity is high enough to trigger the "diamond" reading on thermal testers, these devices cannot distinguish moissanite from diamond. They were not designed to — thermal testers predate the commercial availability of moissanite. See our complete diamond tester guide for buyer-facing guidance.
Why Moissanite Is a Semiconductor
Silicon carbide (SiC) is one of the most important semiconductor materials in electronics and power engineering. Its bandgap of 2.3–3.3 eV (depending on polytype) makes it an excellent semiconductor for high-temperature, high-power, and high-frequency applications. SiC semiconductors are used in electric vehicle power electronics, solar inverters, and industrial motor drives.
This semiconducting property means moissanite conducts electricity — not as well as a conductor like copper, but measurably, unlike diamond, which is a wide-bandgap insulator with a bandgap of 5.5 eV. When an electrical diamond tester applies a small voltage to moissanite, it detects current flow and correctly identifies the stone as "not diamond" — but the device typically displays this as "not diamond" without specifying what the stone actually is.
What Happens at a Jeweler's Counter
A jeweler who uses only a thermal tester will read your moissanite as diamond. A jeweler who uses a combined thermal-electrical tester (the standard tool for experienced jewelers today) will correctly identify that the stone is not diamond — but the device will not tell them it is moissanite. An experienced gemologist who then examines the stone under magnification will see moissanite's characteristic doubling of facet edges (caused by its birefringence) and correctly identify it. See our complete jeweler testing guide for a step-by-step walkthrough of what to expect.
The GRA Certificate: Your Complete Documentation
The most effective response to any testing situation is your GRA certificate — the official gemological document that identifies your stone as moissanite, records its color grade, clarity grade, carat weight, and cut grade, and provides a unique certificate number for verification. A jeweler who questions your stone's identity after seeing a GRA certificate has all the information they need. See our GRA certificate guide and our step-by-step reading guide.
Summary: The Three Physical Properties
Moissanite has three relevant physical properties for testing purposes: high thermal conductivity (causes thermal testers to read "diamond"), electrical semiconductivity (causes electrical testers to read "not diamond"), and optical birefringence (causes doubling of facet edges visible under 10x magnification, conclusively identifying moissanite to any trained gemologist). Together, these properties make moissanite uniquely identifiable — not by a single test, but by the combination of all three.