Moissanite is sold as a diamond alternative, and at a glance in a ring setting the two can look similar. Underneath, they are entirely different substances with different measurable properties — this is a comparison of physics, not of which one anybody should buy.
What each one actually is
Diamond is a single element, carbon, crystallised under enormous heat and pressure. Moissanite is silicon carbide — silicon and carbon together — first identified in 1893 by the chemist Henri Moissan, in tiny fragments recovered from the Canyon Diablo meteorite crater in Arizona. Natural moissanite is vanishingly rare; essentially every moissanite sold as a gemstone today is grown in a lab.
Hardness
On the Mohs scale, diamond sits at 10, the hardest substance the scale measures. Moissanite runs 9.25–9.5, depending on the crystal orientation tested. Both numbers are far above anything a ring encounters in daily wear — the full scale, and why the gap between 9 and 10 is much larger in absolute terms than the gap between 1 and 2, is in the Mohs hardness scale tool.
Why moissanite throws more colour
Two optical properties explain the “rainbow” look people associate with moissanite. Its refractive index, around 2.65–2.69, is meaningfully higher than diamond’s roughly 2.42 — light bends more sharply passing through it. Its dispersion, the measure of how much a material splits white light into spectral colour, is about 0.104, against diamond’s 0.044 — more than double. In direct or bright light, that combination produces noticeably more flashes of spectral colour from a moissanite stone than a diamond of the same cut and size.
Singly refractive against doubly refractive
Diamond’s crystal structure is cubic and optically isotropic, meaning light passes through it as a single ray — it is singly refractive. Moissanite’s crystal structure is hexagonal and birefringent, meaning it splits a ray of light in two as it passes through. In a larger moissanite, this can be seen under 10x magnification as a faint doubling of the pavilion’s facet edges when viewed through the table — a diagnostic a gemologist can use that has nothing to do with colour or brilliance.
Why a diamond tester alone is not reliable
Standard handheld diamond testers work by measuring thermal conductivity, and diamond conducts heat exceptionally well. So, as it turns out, does moissanite — closely enough that a thermal-only tester reads both as “diamond.” In a widely cited 1997 test, forty jewellers and pawnbrokers using thermal probes misidentified synthetic moissanite as genuine diamond. The trade’s response was the dual tester: it still checks thermal conductivity, then adds a second check for electrical conductivity, since moissanite is a semiconductor and will register a small electrical signal that diamond will not. That second step is what actually separates the two reliably, not the thermal reading alone.
What this comparison is not
This page measures two substances against each other. It does not rank them, and it will not tell you which belongs in a ring — that depends on what someone wants from a stone, and this site has no opinion on that. If you are trying to put a size to a stated carat weight in either material, the gemstone carat size chart and diamond size calculator both account for how density changes what a given weight looks like.