Diamond vs moissanite

Diamond is carbon, cubic, Mohs 10, specific gravity 3.52, refractive index 2.417 and dispersion 0.044. Moissanite is silicon carbide, hexagonal, Mohs 9.25, specific gravity 3.21, refractive index roughly 2.65–2.69 and dispersion 0.104. Both are hard and both are singly coloured when pure; the numbers are simply not close.

The two that matter most for identification are dispersion and crystal system. Moissanite disperses more than twice as much light into spectral colours, and it is doubly refractive where diamond is not.

Excess fire is the first clue: moissanite can read as slightly "too colourful" in direct light, with more rainbow flashes per facet than a diamond of the same size — though a hard light source and a phone camera can produce a similar look from a diamond. Facet doubling is the more decisive difference, but it is a magnification effect: it takes a loupe aimed through the stone, and an ordinary photograph rarely resolves it. Under a microscope moissanite also shows characteristic needle-like inclusions that run in one direction.

None of that is diagnostic. Lighting, white balance, compression and the camera's own optics create and destroy the same cues, and a well-cut moissanite photographed in soft light looks entirely diamond-like. A photo can support "worth checking with an instrument" — never "this is a diamond".

A refractometer reads the difference immediately (1.78 is the usual refractometer limit, so neither stone reads directly — the reading is done with a contact liquid or by distinguishing single from double refraction with a polariscope). In practice a jeweller uses a polariscope to show moissanite's doubling, a diamond tester to confirm thermal or electrical behaviour, and a microscope for the inclusion pattern.

Value, treatment and origin need more than identification: lab-grown diamond detection, for instance, is a laboratory procedure even when the stone is unambiguous to the eye.