How to Tell Moissanite From a Diamond

Look for two things: extra rainbow flash, and doubled facet edges. Moissanite disperses light more than twice as strongly as diamond, so it throws noticeably more colour. It is also doubly refractive, so under a 10x loupe angled through the crown you see each back facet edge as two lines. A standard thermal diamond tester will not tell them apart — it reads moissanite as diamond.

Moissanite and diamond are easy to confuse in a photograph and surprisingly easy to separate in person, once you know which two properties differ. Everything below follows from those two.

1. Fire — visible without any tools

Dispersion is how strongly a stone splits white light into spectral colours. Diamond's is 0.044. Moissanite's is 0.104 — well over twice as much. In practice that means moissanite throws distinctly more rainbow flash, particularly under a point light source like a spotlight or direct sun.

Scale matters here. Below about half a carat the difference is subtle enough that most people won't call it. Past a carat it becomes the most obvious tell there is, and it is the reason some people prefer moissanite outright and others don't: the extra colour reads as livelier to some eyes and as less restrained to others.

2. Doubling — the definitive visual test

Diamond is singly refractive: light passes through along one path. Moissanite is doubly refractive, splitting entering light into two rays. The visible consequence is that facet edges appear doubled.

To see it, use a 10x loupe, tilt the stone so you are looking through the crown at an angle rather than straight down the table, and focus on the facet junctions on the far side. In a moissanite, those back edges resolve into two parallel lines. In a diamond they stay single. This is the test gemologists use, and once you have seen it once it is unmistakable.

3. Weight, if you can measure it

Moissanite is less dense than diamond — roughly 3.21 g/cm3 against 3.52. Two stones cut to identical millimetre dimensions will therefore differ in carat weight, with the moissanite around 10 to 12 percent lighter. This only helps if you can weigh a loose stone and measure it accurately, so it is a jeweller's method rather than a shopper's.

4. Testers — and the one that misleads

The pen-style testers most people have seen measure thermal conductivity. Diamond conducts heat exceptionally well, which is what the tester looks for. So does moissanite. A thermal tester reads moissanite as diamond, and this is the single most common source of a wrong identification.

The fix is a combination tester that also measures electrical conductivity. Moissanite conducts electricity; diamond does not. A dual tester separates them reliably. If someone tells you a stone tested as diamond, it is worth asking which kind of tester was used.

A note on lab-grown diamond

None of the above applies to lab-grown diamond, which is chemically and optically a diamond and cannot be told from a mined one by eye, by loupe, or by any handheld tester. Distinguishing those two requires laboratory equipment that reads growth signatures, which is precisely why reputable lab-grown stones are certified and laser-inscribed.