Moissanite is the simulant that breaks people's trust in their own testing kit. It's brilliant, durable, and — crucially — it conducts heat almost like diamond. That last property is why the $20 tester pen in your drawer is quietly unreliable.
What moissanite actually is
Moissanite is silicon carbide (SiC), originally discovered in a meteor crater. Today it's lab-created and sold as an affordable, ethical diamond alternative. It is not a diamond and not trying to secretly be one in the legitimate market — but in the resale and private-sale market, it's the stone most often passed off as the real thing.
Why thermal pens fail
Standard "diamond testers" measure thermal conductivity. Diamond scores extremely high — and so does moissanite. A thermal-only pen will happily light up green for both. If a seller proves their stone with a single pen, they've proven nothing about moissanite.
What actually separates them
- Fire: Moissanite's dispersion (0.104) is more than double diamond's (0.044). Under bright light it flashes far more rainbow colour.
- Double refraction: Look through the table at the back facets under a loupe. Moissanite is doubly refractive — facet edges appear slightly doubled. Diamond is singly refractive and shows crisp single lines.
- Electrical conductivity: Moissanite conducts electricity; diamond doesn't. A combined tester nails this instantly.
- Refractive index: 2.65 (moissanite) vs 2.42 (diamond) — a definitive instrument reading.
Why it matters
A one-carat natural diamond can run several thousand dollars. A visually similar moissanite costs a fraction of that. If you're buying second-hand, inheriting jewellery, or selling, mistaking one for the other is a four-figure error. Don't let a single pen make that call.
Stop guessing. Get an expert verdict on your own stone in minutes.
Written by
Dr. Helena Voss
Chief Gemologist, GIA GG
Part of the Diamonds Tester gemology team — combining lab-grade instruments with decades of grading experience to give every stone a straight, honest verdict.