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Moissanite vs lab diamond: which to buy and why

Updated

For an engagement ring, choose a lab-grown diamond if you want real diamond with an IGI report that tests as diamond; choose moissanite if you want the biggest, most sparkly stone for the least money. Moissanite is , not diamond. It is a little softer (9.25 against 10 on the scale) and throws more than twice the rainbow fire. A basic thermal tester calls both "diamond"; an electrical tester or a 10x loupe tells them apart.

Moissanite and lab diamond side by side

Lab-grown diamond vs moissanite: material, hardness, optics, testers, paperwork and resale
Lab-grown diamondMoissanite
MaterialDiamond: carbon, the same material as a mined diamondSilicon carbide, a different mineral. Natural moissanite is extremely rare and tiny, so every gem moissanite is grown
Mohs hardness10, the hardest9.25, the hardest of the diamond look-alikes
Refractive indexAbout 2.42About 2.65 to 2.69 (two values, because it is doubly refractive)
Dispersion (rainbow fire)0.0440.104, more than twice the fire of diamond
Under a 10x loupeSingly refractive: back facet edges look crispDoubly refractive: back facet edges look doubled when you look through the stone at an angle
Thermal "diamond" testerReads diamondAlso reads diamond
Electrical or combined testerDoes not conduct, so reads diamond. Exception: blue boron-bearing diamonds, natural or lab-grown, conductConducts, so the tester flags it as moissanite
PaperworkAn IGI report with colour, clarity and cut grades, or a GIA assessment ("Premium" or "Standard" since 1 October 2025). The report number can be looked up on the lab's websiteUsually a GRA card that comes from the seller. Treat its grade as the seller's claim, not a lab report
ResaleLow, and falling: wholesale prices fell 26% in 2025 on Edahn Golan's index, which is down 96% since 2018Expect little back. Buy it to wear, not to resell

Diamond hardness and : the US FTC definition of diamond (16 CFR 23.12). Moissanite hardness, refractive indices and dispersion, and the thermal tester result: GIA, Gems & Gemology, Winter 1997; "more than twice the fire" and "hardest of the diamond simulants": GIA 4Cs. Diamond dispersion: Wikipedia. Resale: JCK, 10 August 2026. Prices change daily, so we do not print them here; compare current listings on our lab diamond and moissanite pages.

moissanite's coloured flashes (left) beside diamond's whiter brilliance (right)
Illustration (AI render): moissanite's coloured flashes (left) beside diamond's whiter brilliance (right).

Is moissanite a diamond?

No. Moissanite is silicon carbide; diamond is carbon. GIA lists synthetic moissanite among the diamond simulants: stones that look like diamond but do not share its chemical, physical and optical properties (GIA 4Cs). Natural moissanite is vanishingly rare, tiny grains in meteorites and a few deep-earth rocks; every gem you can buy is grown. It has a higher refractive index than diamond (about 2.65 against 2.42) and much higher dispersion, which is the rainbow flash. Lab diamond is chemically, optically and physically diamond; the only differences are growth marks a laboratory can read, and the price.

The grades on the listing

Amazon moissanite listings borrow the diamond scale: D colour, VVS1 clarity. The paperwork is usually a GRA card, which most moissanite listings include. It is a seller-supplied card, not a report from an independent laboratory; treat its grade as the seller's claim. When the Gemmological Association of Australia bought a parcel of twenty moissanite stones online, it found their GRA cards generic and virtually identical apart from the number, modelled on GIA reports and carrying cut grades that are not applied to moissanite (Australian Gemmologist, 2023). On lab diamond, an IGI report with a matching girdle inscription gives the full colour and clarity grade. GIA no longer grades lab-grown diamonds on its D-to-Z colour and FL-to-I3 clarity scales: since 1 October 2025 it gives a "Premium" or "Standard" designation, or no assessment below Standard (GIA, 26 August 2025; GIA, 2 June 2025). Melee and other small stones are usually sold without a report. How to look a report number up: checking a diamond report number; what a GRA card is: is a GRA certificate real?

Comparison chart of lab diamond, moissanite and cubic zirconia: hardness 10 vs 9.25 vs 8 to 8.5, refractive index, fire, look, daily wear and how each reads on a diamond tester
Lab diamond, moissanite and cubic zirconia side by side. Free to save and share.

How can you tell them apart?

A thermal diamond tester reads both as diamond (GIA, 1997). An electrical tester usually separates them: moissanite conducts, diamond does not, though rare blue boron-bearing diamonds (and some lab diamonds grown with boron) conduct too: boron makes both natural and lab-grown type IIb diamonds conduct electricity, and no other type of diamond does (Gems & Gemology, Summer 2009). Under a loupe, moissanite is doubly refractive, so the back facets look doubled when you look through the stone at an angle; diamond is singly refractive and the facets look crisp. When a moissanite carrying a fake GIA inscription reached GIA's laboratory, its obvious doubling under the microscope helped give it away (Gems & Gemology, Fall 2020). A combined thermal and electrical tester settles it in seconds; see our gem tools and does moissanite pass a diamond tester?

Which is better for an engagement ring?

If you want a diamond, buy a lab-grown diamond with an IGI report or a GIA assessment: it is diamond, it tests as diamond, and the report number on the girdle ties the stone to its paperwork. If you want the biggest, brightest white stone for your budget and are happy that it is not a diamond, buy moissanite: at 9.25 it is hard enough for a ring worn every day.

Frequently Asked Questions

Will moissanite pass a diamond tester?

A thermal tester, yes: moissanite conducts heat far better than other diamond look-alikes, enough to read as diamond. An electrical tester or a combined tester separates them, because moissanite conducts electricity and diamond does not. Jewellers use the combined kind for this reason.

Does moissanite look fake?

In sunlight a large moissanite throws more rainbow flash than a diamond, which some people love and some read as too much. Under a carat, most people cannot tell. Colour has improved: older moissanite had a green or yellow tint; current stones sold as D to F colour are near colourless.

Which is harder?

Diamond, at 10 on the Mohs scale; moissanite is 9.25, harder than sapphire at 9. Both scratch glass and shrug off daily wear; both can chip on a hard knock at a thin girdle.

Sources