Why does moissanite test positive on a diamond tester?

Standard diamond testers measure thermal conductivity. Moissanite (silicon carbide) has a thermal conductivity of approximately 490–500 W/m·K — far higher than any other gemstone simulant and in the same range as diamond. When the tester detects this rapid heat dissipation, it signals 'diamond.' The tester is working correctly; it simply cannot distinguish between the two thermally conductive materials.

How can you tell moissanite apart from diamond?

The most reliable methods are: (1) A dual tester that measures both thermal and electrical conductivity — moissanite is a semiconductor and conducts electricity; diamond is an insulator and does not. (2) GRA certification, which independently confirms the stone's identity. (3) Optical examination under magnification — moissanite's double refraction creates a characteristic doubling of facet edges visible under a loupe.

Is moissanite's thermal conductivity similar to diamond?

Moissanite has a thermal conductivity of approximately 490–500 W/m·K. Diamond ranges from 1,000–2,600 W/m·K depending on type. Both values are orders of magnitude higher than all other gemstones (cubic zirconia: ~2 W/m·K, quartz: ~7 W/m·K, sapphire: ~27 W/m·K). This is why standard diamond testers cannot distinguish moissanite from diamond.

Does moissanite's high thermal conductivity affect how it wears?

Yes — positively. Moissanite's high thermal conductivity means it dissipates heat rapidly, reducing thermal stress on the crystal structure from sudden temperature changes. This contributes to its exceptional fracture toughness (7.6 MPa·m¹⁄², compared to diamond's 3.4 MPa·m¹⁄²) and its stability in all environments encountered in daily jewelry wear.

Moissanite Thermal Conductivity: Why Diamond Testers React

September 22, 2026

If you have ever placed a moissanite stone under a standard diamond tester and watched it register as diamond, you have observed one of moissanite's most remarkable material properties: thermal conductivity nearly equivalent to diamond, and far exceeding every other gemstone. This guide explains why it happens and what it means.

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How Standard Diamond Testers Work

A standard diamond tester — the pen-like device used by jewelers, pawnshops, and gemologists worldwide — operates on thermal conductivity. The device applies a small amount of heat to the surface of the stone through a metal tip, then measures how quickly that heat dissipates through the material.

Diamond is an extraordinary thermal conductor — it dissipates heat faster than virtually any other material at room temperature. When a standard diamond tester detects rapid heat dissipation, it signals "diamond." Materials that dissipate heat slowly — glass, cubic zirconia, quartz, most simulants — register as "not diamond."

Moissanite's Thermal Conductivity

Moissanite (silicon carbide, SiC) has thermal conductivity of approximately 490–500 W/m·K — comparable to natural diamond (which ranges from 1,000–2,600 W/m·K depending on type and purity). Both values are orders of magnitude above all other gemstones:

Material	Thermal Conductivity (W/m·K)
Diamond	1,000–2,600
Moissanite	490–500
Sapphire	~27
Quartz	~7
Cubic Zirconia	~2

When a standard diamond tester's tip contacts a moissanite surface, it detects rapid heat dissipation in the diamond range and signals "diamond." The tester is working correctly — it simply cannot distinguish between the two thermally conductive gemstones.

The Dual Tester Solution

The gemological response to moissanite's thermal conductivity was the development of the dual tester — a device that tests both thermal conductivity and electrical conductivity to distinguish diamond from moissanite.

Moissanite is a semiconductor — it conducts a small amount of electricity
Diamond is an electrical insulator — it does not conduct electricity

A dual tester applies both tests: a stone that passes the thermal test and also conducts electricity is identified as moissanite. A stone that passes the thermal test and does not conduct electricity is identified as diamond.

Any reputable gemological laboratory or well-equipped jeweler can correctly identify moissanite from diamond with a dual tester in seconds. GRA certification independently confirms the stone's identity without requiring any tester. See: GRA Certificate Explained: How to Verify Your Moissanite →

What This Property Means in Practice

For buyers: A standard diamond tester is not a reliable tool for distinguishing moissanite from diamond — it will register moissanite as diamond. If you want to verify a stone's identity, use a dual tester or request GRA certification.

For moissanite owners: If a standard tester at a jeweler or pawnshop registers your moissanite as diamond, this is expected and does not require correction unless the context requires precise identification.

For gemological purposes: Moissanite's semiconductor property under electrical testing is the definitive distinction from diamond. No other common gemstone simulant shares moissanite's combination of thermal and electrical properties.

Thermal Conductivity and Stone Care

Moissanite's high thermal conductivity has a practical benefit for durability: it dissipates heat rapidly, reducing thermal stress on the crystal structure from sudden temperature changes. This contributes to moissanite's exceptional toughness (fracture toughness 7.6 MPa·m¹⁄²) and its stability across all environments a piece of fine jewelry encounters in daily wear.

The Broader Material Picture

Moissanite's thermal conductivity is one of several properties that position it as the closest material analog to diamond in the gemstone world:

Property	Moissanite	Diamond
Mohs Hardness	9.25	10
Thermal Conductivity (W/m·K)	490–500	1,000–2,600
Refractive Index	2.65–2.69	2.417
Dispersion (Fire)	0.104	0.044
Fracture Toughness (MPa·m¹⁄²)	7.6	3.4

In hardness, thermal behavior, and optical performance, moissanite occupies a category shared only with diamond — and in dispersion and refractive index, it exceeds it. For more on the science: Moissanite vs. Diamond: The Ultimate Comparison Guide →