The question contains the confusion. There is no such thing as a moissanite diamond, and the phrase is doing a lot of quiet work in a lot of product listings.
Here is the straight answer in two parts. Moissanite is a real gemstone. Moissanite is not a diamond. Both are true at once.
Real, in the sense that matters
Moissanite is silicon carbide, formula SiC. It is a genuine mineral with its own hardness, its own optical behaviour and its own history. It is not glass, not plastic, not a coating over something cheaper. Under a loupe it is unmistakably a cut gemstone.
It is also grown in a laboratory, and that is not a caveat. Natural moissanite exists only in some meteorites and a few rare rocks, in grains far too small to cut. Every moissanite in every piece of jewellery on earth was grown. There is no other source, so there is nothing being substituted for anything.
Not a diamond, and not trying to be
Diamond is carbon. Moissanite is silicon carbide. Different elements, different crystal structure, different behaviour in light. Calling one the other is like calling silver a kind of platinum.
A gemmologist separates them in seconds. Moissanite is doubly refractive, which means it splits light into two paths. Look down through the top facet under ten times magnification and the far edges appear doubled. Diamond is singly refractive and shows them single. That test is over a century old and has not stopped working.
Where the phrase came from
Search for moissanite and you will find listing after listing saying moissanite diamond, and often adding a colour and clarity grade that looks like a diamond report.
That language exists because it sells. It borrows the authority of one stone for another. D to Z is the GIA colour scale, written for diamonds. VVS is the GIA clarity scale, also written for diamonds. No recognised laboratory grades moissanite on either, so when a listing states those grades it is the manufacturer describing its own product rather than reporting an independent result.
It is not necessarily false. Good colourless moissanite really is colourless. But self-declared is not the same as certified, and a seller who blurs that is telling you something about how they will behave later.
The diamond tester question
Moissanite passes most handheld diamond testers, and this is the fact that makes people uneasy.
The reason is mundane. A basic tester does not examine a stone. It touches it with a warmed probe and measures how fast the heat leaves. Diamond conducts heat better than almost anything, and silicon carbide happens to conduct it almost as well. The probe cannot tell them apart.
Modern dual testers add an electrical probe. Moissanite conducts electricity and diamond, in almost every case, does not, so a dual tester names it correctly. Between the loupe and the dual tester, moissanite is not hard to identify. Anyone promising it is undetectable is selling you a story.
Where moissanite is genuinely better
Two measures, and they both favour moissanite.
Refractive index: moissanite 2.65 to 2.69, diamond 2.42. More light returns to your eye.
Dispersion, the splitting of white light into colour: moissanite 0.104, diamond 0.044. Two and a half times as much fire.
In daylight a moissanite throws noticeably more colour than a diamond the same size. Some people love that and some find it too much. It is a question of taste, not of quality, and you should know it before buying rather than after.
How to buy one honestly
- Ask for millimetres, not carats. Moissanite is lighter than diamond, so a stone sold as one carat is either wider than a one carat diamond or weighs about 0.88
- Ask what the setting metal actually is, since that is what tarnishes and wears, not the stone
- Treat colour and clarity grades as the seller's own description
- Treat any certificate as a receipt rather than an independent grading report
What we do
We sell moissanite necklaces and pendants in 925 sterling silver beneath a gold-tone finish, and we call the stone moissanite everywhere it appears. What our supplier states about materials we pass on exactly as given. What we have not tested ourselves, we do not present as proven.
The stone passes a diamond tester, and this house still calls it moissanite.