Set a moissanite and a lab grown diamond side by side on a counter and most people can't tell which is which. Search for them online and it gets more confusing, because half the listings call moissanite a diamond (it isn't) and the other half call lab grown diamonds fake (they aren't either).
The difference isn't a matter of opinion or marketing. One of these stones is a diamond. The other is a different mineral that happens to look like one.
Carbon versus silicon carbide
A diamond is carbon. Moissanite is silicon carbide, chemical formula SiC. GIA is blunt about simulants: the common ones are made from glass, moissanite and cubic zirconia, and they have nothing to do with diamond at the atomic level. It also describes colourless synthetic moissanite as a diamond imitation that caught on in the late 1990s.
Lab grown diamonds are a different story. GIA says diamonds grown in a laboratory have essentially the same chemical composition, crystal structure and physical properties as natural ones. IGI is more direct still, calling a certified laboratory grown diamond a real diamond with the same chemical, physical and optical properties as a mined one. What separates the two is where they came from and how long they took. According to GIA, a lab grown stone forms in days or weeks. A natural diamond formed millions to billions of years ago.
That's why the terminology matters. CIBJO, the international jewellery confederation, defines a diamond simulant as an artificial product that imitates the look of a diamond without having its chemical composition, physical properties or structure. Moissanite fits that definition exactly, so "moissanite diamond" isn't loose wording, it's simply wrong. The correct terms are moissanite, or imitation diamond. For stones grown in a laboratory, the accepted terms are laboratory grown diamond, laboratory created diamond or synthetic diamond, and all three mean a genuine diamond.
The full science on the lab grown side is in our piece are lab grown diamonds real.
How each one is made
There are two ways to grow a diamond in a lab. High pressure high temperature growth uses equipment that recreates the pressure and heat under which diamonds form in the earth, and the crystals come out with flat bases and octahedral and cube faces. Chemical vapour deposition breaks down carbon-containing gases inside a vacuum chamber, and the carbon crystallises onto diamond seed plates. Those crystals are tabular and sometimes edged with graphite. We go through both in more detail in how lab grown diamonds are made.
Moissanite has an odder backstory. The French chemist Henri Moissan found silicon carbide crystals in material from the Canyon Diablo meteorite in Arizona, and at first he thought they were diamond. Natural moissanite does exist, but not in any form you could set in a ring. GIA reports that it's very rare and turns up as tiny crystals, usually under 1.5 millimetres, in only a few deposits worldwide. The largest specimen documented as of 2012 measured 4.1 millimetres. So every moissanite in a ring is lab grown silicon carbide. There's no mined version to choose instead.
What the numbers say about how they behave
Once you know what to look for, three measurements show where the two stones part company.
Hardness
Diamond is 10 on the Mohs scale, the highest hardness there is. GIA gives moissanite 9.25. That still makes it one of the hardest materials you can wear, well above sapphire. But the Mohs scale isn't linear, and diamond is meaningfully harder. On a ring you'll wear every day for decades, that gap is real.
Fire
Dispersion is what splits white light into spectral colours, and it's what people mean when they say a stone throws rainbows. Diamond's dispersion is 0.044. GIA gives moissanite 0.104, roughly two and a half times as much.
This is the biggest visual giveaway. A moissanite of any size shows noticeably more coloured flash than a diamond of the same size and cut. Some people love that. Others feel it looks obviously not like a diamond, particularly in larger stones under bright light. Both are fair reactions, but look at the two in daylight before you decide which camp you're in.
Refraction
Diamond is singly refractive, with a refractive index of 2.417. Moissanite is doubly refractive, so it has two indices, 2.648 and 2.691. GIA spells out what that means in practice: you see a double image of the back facets. Look down through the top of a moissanite under magnification and the junctions between the lower facets appear doubled. In a diamond they don't.
For a trained eye, this is the cleanest way to tell the two apart. GIA has described moissanite as readily separated from diamond on the basis of that doubling ever since the material first came on the market.
Why a diamond tester won't save you
This is the most expensive misunderstanding in the whole subject.
The pen-style testers most people have seen work by measuring thermal conductivity. Diamond conducts heat unusually well, so a stone that behaves like diamond thermally lights the tester up. Moissanite defeats that logic. When synthetic moissanite reached the market, GIA reported that its thermal properties were so close to diamond's that the thermal probes available at the time reacted to it as if it were diamond. You need a more advances diamond-moissanite tester to tell them apart.
So a basic thermal tester will happily tell you a moissanite is a diamond. If someone at a market stall waves a pen over a stone and declares it genuine, the demonstration proves nothing. Magnification will show the facet doubling, and where there's any doubt, proper gemmological testing settles it. We cover checking a stone more broadly in how to check if your diamond is real.
Certification is where lab grown diamonds pull ahead
Lab grown diamonds are graded by the same laboratories, on the same scales, as mined ones. GIA issues a Laboratory-Grown Diamond Report. It looks different from the natural diamond report, with a grey border instead of a gold one, but the grading standards are the same. GIA also laser inscribes the girdle with the report number and the words "laboratory-grown", so the stone's origin stays with it for good. IGI grades lab grown diamonds on the same core principles it applies to natural stones (cut, colour, clarity and carat weight) and its report is visually distinct too.
Moissanite isn't graded on that system. It isn't a diamond, so the diamond grading scales don't apply to it.
That matters when you're buying. A certified lab grown diamond comes with an independent, documented description of exactly what you own, marked permanently on the stone. With moissanite, you're relying on what the seller tells you. Our guide to choosing a lab grown diamond explains how to read those reports.
On price and resale, be sceptical of anyone quoting figures
You'll find plenty of confident percentages online about how lab grown diamonds are priced against mined ones, and what either is worth once you've bought it. We're not adding to them. When we went looking for a figure from an institution rather than a shop, we couldn't find one. GIA, IGI, the Gemmological Association and CIBJO publish no pricing or resale data for lab grown diamonds. Every percentage going around traces back to a retailer or a piece of marketing.
What we can say honestly is that prices in this market move, that moissanite and lab grown diamonds are both produced rather than found, and that you shouldn't buy either expecting it to hold its value. Buy the stone because you want to wear it. We make the longer argument in are lab grown diamonds a good investment, and compare the two kinds of diamond in lab grown versus natural diamonds.
What you're owed at the point of sale
CIBJO's Diamond Blue Book requires full verbal disclosure, in clear and understandable language, before a sale is completed. It also requires full written disclosure, included conspicuously on all commercial documents. That's an international trade standard rather than an Irish statute. Still, a seller who passed off a moissanite or a lab grown stone as a mined diamond would also be breaking general EU and Irish law on misleading commercial practices.
In practice, you should never have to work out for yourself what a stone is. Ask, and then ask for the answer in writing on the receipt. If a seller gets vague about which of the three you're buying, that's worth noticing.
So which should you choose?
If what you want is a diamond, a lab grown diamond is one and a moissanite isn't. Looking similar doesn't change that.
If you want a hard, bright, colourless stone with plenty of fire and you don't mind that it's a different material altogether, moissanite is a perfectly legitimate choice. It's hard enough for daily wear and has more fire than diamond. Just buy it knowing exactly what it is, and don't pay diamond money for it.
The mistake to avoid is ending up with something sold as a diamond and priced like one that turns out to be silicon carbide. Ten seconds under a loupe with someone who knows what facet doubling looks like will tell you which you're holding.
If you're weighing up a stone and want an honest opinion on it, call in and talk to us. You can also see what we have on our engagement rings page.