Side by side
| Property | Diamond | Moissanite |
|---|---|---|
| Mohs hardness | 10 | 9.25 |
| Refractive index | 2.42 | 2.65 |
| Dispersion (fire) | 0.044 | 0.104 |
| Specific gravity | 3.52 | 3.22 |
| Thermal conductivity | Exceptional | Very high |
| Conducts electricity | No | Yes |
| Double refraction | No | Yes |
| Rough price per carat | Thousands | Hundreds |
Why every home test fails
Run down the usual list. The water test: moissanite is dense and sinks. The fog test: it conducts heat almost as well as diamond, so fog clears in about a second. The read-through test: its refractive index is actually higher than diamond, so text is even more thoroughly blocked. Scratching glass: it is 9.25, it scratches everything you own.
That is not a coincidence. Moissanite was developed as a diamond simulant precisely because it matches diamond on the properties people use to check.
The three things that do give it away
Doubling under a loupe. This is the one to trust. Moissanite is doubly refractive, so look through the crown at a slight angle at ten power magnification and the facet edges on the opposite side of the stone appear as two lines slightly offset. Diamond never does this. Once you have seen it, it is unmistakable.
Too much fire. Moissanite has more than twice the dispersion of diamond, so it throws noticeably more rainbow, especially in direct light. Some people describe it as looking slightly oily or disco-like. On its own this is subjective, but paired with doubling it settles things.
Electrical conductivity. The decisive test, and the reason modern jewellers carry combination testers. Moissanite conducts electricity, diamond does not. It takes about four seconds and any jeweller will do it, usually for free.
If you suspect moissanite
Go to a jeweller. This is the one case where home testing genuinely runs out of road, and a combination tester answers it definitively in seconds. Do not spend an evening working through internet tests that were all designed before moissanite was common.
Not sure what you are looking at?
Photograph the stone and the app gives you a visual read on the 4Cs, an estimated value range, and what to check next. Free on iOS and Android.
Frequently asked questions
Why does moissanite fool diamond testers?
Is moissanite a fake diamond?
What is doubling and how do I see it?
Which is harder?
Worth being clear: everything here, and the app itself, is an educational reference. At-home tests narrow things down, they do not certify a stone. Before you buy, sell or insure anything of real value, get it looked at by a certified gemologist or an independent appraiser.