Moissanite vs Diamond: How to Tell the Difference

Moissanite and diamond can look alike in a ring, but they are different materials. Diamond is carbon. The FTC Jewelry Guides define it as a mineral consisting essentially of pure carbon crystallized in the isometric system, with a hardness of 10, a specific gravity of about 3.52, and a refractive index of 2.42. Moissanite is silicon carbide, and the moissanite sold in jewelry is made in a laboratory. It is a diamond simulant: a different material that imitates diamond, not a laboratory-grown diamond.

The practical differences are small to the eye and large on an instrument. Moissanite shows more spectral dispersion, is doubly refractive, is slightly less hard, and weighs less for the same size. It also fools the basic thermal testers that many people call diamond testers. A loupe in trained hands, or a gemological laboratory, separates the two reliably.

Moissanite vs diamond at a glance

The diamond values are from the FTC definition and GIA. The moissanite values are from the GIA study "Synthetic Moissanite: A New Diamond Substitute," published in Gems & Gemology in 1997, which examined the material shortly before it was released to the jewelry trade.

What moissanite is

Silicon carbide has been made industrially for more than a century, first as an abrasive. GIA's study recounts that Edward G. Acheson made it while trying to grow diamond, and that the chemist Henri Moissan later found natural silicon carbide in the Canyon Diablo meteorite. The mineral was named moissanite in his honor. Natural moissanite has been reported only in tiny amounts, so the stones in jewelry are synthetic.

Near-colorless synthetic moissanite for jewelry became possible only once its crystal growth could be controlled. GIA described it as much closer to diamond in overall appearance and heft than any earlier diamond imitation. That is what makes the comparison worth knowing. GIA described every earlier imitation as having significant deficiencies, such as the lower brilliance of synthetic spinel or the high density of cubic zirconia, while moissanite is close to diamond on several of the tests people rely on.

A laboratory-grown diamond is a different case. It is diamond, with the composition and properties of a mined diamond, made by a manufacturing process. Moissanite is not diamond in any form, natural or grown. For lab-grown diamond reports and terms, see lab-grown diamonds: what you need to know.

Fire and sparkle

Dispersion is the spread of white light into spectral colors, the flashes jewelers call fire. GIA measured moissanite's dispersion at 0.104, more than twice diamond's 0.044, and its refractive indices of 2.648 and 2.691 are higher than diamond's 2.417. On paper, moissanite should show more colored fire.

What you actually see depends on the cut, the size of the stone, the lighting, and how you look at it. In GIA's 1997 examination, orange and blue flashes were sometimes visible on moissanite pavilion facets but were much less distinct than the similar colors seen on cubic zirconia. GIA's own guidance on telling real diamonds from imitations makes the wider point: to an untrained eye, almost any faceted gem sparkles. Fire and sparkle are reasons to look closer, not identification tests.

Double refraction: the most useful visual clue

Diamond is singly refractive. Moissanite is doubly refractive, so light passing through it splits into two rays. GIA calls this double refraction the most readily identifiable gemological property of near-colorless moissanite.

To see it, look into the stone at an angle through the crown facets with a 10x loupe. The edges of the pavilion facets on the far side appear doubled, like a slightly out-of-register print. Cutters usually orient moissanite so that the direction without doubling runs straight down through the table, so looking face-up through the table can show nothing. Tilt the stone, or focus past the point at the bottom of the stone, the culet, to look for doubled reflections. In larger stones GIA noted that the doubling can sometimes be seen with the unaided eye.

Doubling rules out diamond, but it does not prove moissanite. Other doubly refractive simulants, such as synthetic rutile, show doubling too. GIA's study also found other features in the stones it examined: whitish needle-like inclusions, rounded facet junctions where diamond is sharp, and polish lines. It noted that samples received later in the study were better polished, so the finish of a modern stone is a weaker clue than the doubling.

Hardness and durability

Diamond is 10 on the Mohs scale and moissanite is 9¼. GIA cautions that the Mohs scale is compressed at the top: on a linear indentation scale, the gap between corundum at Mohs 9 and moissanite is larger than the gap between corundum and topaz at Mohs 8. Moissanite is far harder than most gems, and diamond is harder still. For how the scale works, see the Mohs scale guide.

Hardness is resistance to scratching, not to breaking. In GIA's table, moissanite's toughness is rated excellent and diamond's good to excellent. When GIA struck a piece of moissanite rough until it shattered, the fragments showed conchoidal fracture and no cleavage, and the authors noted that the girdle of a faceted stone could chip under hard wear. Neither stone is indestructible, and a chipped girdle is worth recording on any stone before you buy.

Weight: same size, different carat weight

Moissanite is less dense than diamond: GIA measured a specific gravity of 3.22, against about 3.52 for diamond. Two round brilliants with the same diameter and proportions therefore weigh different amounts. GIA's example is a 6.50 mm round brilliant diamond of about 1.00 carat, beside a moissanite of the same diameter and cut at about 0.91 carat.

That matters when you compare listings. Compare stones by their measured dimensions, not by a carat figure that may describe a different material. Laboratories can use density as well: in a heavy liquid with a specific gravity of 3.32, moissanite floats while diamond sinks. That is a laboratory procedure, not a home test.

Why a thermal diamond tester calls moissanite a diamond

The common handheld diamond tester is a thermal probe: it responds to how quickly a stone conducts heat away from its tip. Diamond is an exceptional heat conductor. HyperPhysics lists it at about 1,000 watts per meter-kelvin, more than twice the figure for silver, the best-conducting metal. Most simulants conduct heat far less well, which is why thermal probes became a common screen.

Moissanite defeats it. GIA reported that the thermal properties of moissanite are so close to diamond's that the thermal probes on the market reacted to it as if it were diamond. Every moissanite in GIA's study registered as diamond on every thermal instrument tested, because the thermal ranges of the two materials overlap. A seller who says a stone passed a diamond tester has told you only that it is diamond or something that behaves like diamond under heat.

GIA has since documented a harder case: a synthetic moissanite coated with a thin diamond film to imitate a rough diamond. Thermal conductivity testing indicated diamond, and GIA noted that even a very thin coating tested positive. The stone was identified through its needle-like inclusions, X-ray imaging that revealed the coating, and chemical analysis of the surface.

Electrical conductivity and moissanite testers

Electrical conductivity is a better separator, with limits. In GIA's study, 13 of the 23 main moissanite samples showed some electrical conductivity, unlike most near-colorless diamonds. GIA also noted that some light gray or blue diamonds are electrically conductive. A tester that flags conductive stones as moissanite can therefore misread a natural blue diamond, and a nonconductive moissanite needs another check.

The first instrument built only to separate moissanite from diamond worked differently. GIA tested it in 1997: it measured near-ultraviolet transparency, which diamond transmits and moissanite absorbs, and it was designed to be used only after a thermal probe had already said diamond. Most other near-colorless materials also gave a diamond response on it, which is why the order of testing mattered. Any tester used this way is a screen. It can flag a stone for closer examination, but it is not a laboratory identification, and the diamond-coated moissanite GIA described gave mixed electrical readings across its surface.

How laboratories separate them

A gemological laboratory combines several observations. GIA's 1997 study listed the main ones: doubled facet junctions under magnification, a characteristic absorption below about 425 nm in the visible spectrum instead of the cape lines common in near-colorless diamond, infrared and Raman spectra that differ from diamond's, and silicon in an X-ray fluorescence analysis. Moissanite is also less transparent than diamond to ultraviolet radiation and to X-rays.

Small stones are where mistakes hide. In 2015, GIA's New York laboratory examined a colored diamond bracelet set with 162 small round brilliants. The near-colorless stones were screened with an automated instrument, the referred stones were checked under the microscope, and two showed obvious doubling of the facet junctions. Raman spectroscopy confirmed them as synthetic moissanite. GIA noted that melee-sized imitations are being mixed into parcels of natural diamonds, and that small stones are usually not screened one by one.

GIA's consumer advice is direct: there is no reliable at-home test, and the dependable route is a trained gemologist or an independent gemological laboratory, which can separate natural diamonds, laboratory-grown diamonds, and simulants. If a stone already has a grading report, check the report number on the issuing laboratory's own database. See GIA, AGS, and other grading labs for how reports differ.

What the FTC requires sellers to say

The FTC Jewelry Guides set how these stones may be described in the United States.

• The word diamond: using the unqualified word diamond is deceptive for anything that does not meet the definition of diamond, essentially pure carbon crystallized in the isometric system. Moissanite does not meet it.
• Simulants and laboratory-grown stones: naming a gemstone to describe a laboratory-grown, laboratory-created, synthetic, imitation, or simulated stone requires a qualifier such as laboratory-grown, laboratory-created, imitation, or simulated, immediately before the name and with equal conspicuousness. The Guides add that faux is not an adequate disclosure.
• No borrowed identity: words such as laboratory-grown, laboratory-created, or synthetic may be used with the name of a natural stone only when the product has essentially the same optical, physical, and chemical properties as the stone named. Moissanite can be sold as moissanite, or as laboratory-created moissanite. It cannot be sold as a laboratory-grown diamond.
• Real and genuine: the Guides treat real, genuine, natural, precious, and similar words as deceptive when used for a product that is manufactured or produced artificially.

Laboratory-grown diamonds have their own rule. They may be called cultured only with a clear disclosure, such as laboratory-grown or laboratory-created, that the stone is not mined.

Buying cautions

• Get the identity in writing. The invoice should say natural diamond, laboratory-grown diamond, or moissanite. "Diamond alternative" or a brand name is not a material.
• Do not rely on "it passed the diamond tester." A basic thermal tester reads moissanite as diamond. Ask what kind of tester was used and treat any tester as a screen.
• Verify reports at the source. A report number should be checked on the issuing laboratory's website, and the stone's measurements should match the report.
• Check pavé and accent stones. GIA has found moissanite among small diamonds in finished jewelry. A report on a center stone says nothing about the melee around it.
• Use the return window. For a consequential purchase, have an independent gemologist examine the piece while you can still return it.
• Compare by size, not carat weight. A moissanite and a diamond of the same diameter weigh different amounts.

For online purchases, the guide to checking a jewelry listing covers seller checks, photographs, and return windows in more detail.

Home checks and their limits

Our guide to how to tell if a diamond is real explains why fog, water, newspaper, and sparkle checks cannot prove that a stone is diamond, whatever the simulant. For moissanite specifically, the one observation worth making at home is a careful look for doubled facet edges with a 10x loupe, viewed at an angle through the crown. Seeing doubling is a reason to have the stone examined. Not seeing it proves nothing, because the stone may be oriented to hide it or the doubling may be hard to see in a small stone.

What a photo can and cannot show

A clear photo can record the cut, the setting, the marks on the metal, and visible damage. It cannot measure dispersion, refractive index, or thermal response, and it rarely shows facet doubling clearly. Use the ring identifier by picture to document a ring and the jewelry identifier by photo to organize what a picture shows. When the stone's identity matters, follow the photo with a gemologist's examination.

Can a jeweler tell moissanite from a diamond?

A trained jeweler or gemologist can usually separate them with a loupe or microscope, because moissanite is doubly refractive and diamond is not. A laboratory confirms the identification with spectroscopy when needed. A basic thermal tester alone cannot do it.

Does moissanite pass a diamond tester?

It passes the common thermal type. GIA found that every moissanite it tested registered as diamond on thermal probes, because the two materials conduct heat similarly. Testers that add an electrical or ultraviolet reading can flag moissanite, but they are screens, not identification.

Is moissanite a lab-grown diamond?

No. Moissanite is silicon carbide, a different material from diamond. A laboratory-grown diamond is diamond made in a laboratory. Under the FTC Jewelry Guides, moissanite cannot be described as a laboratory-grown diamond.

Does moissanite sparkle more than diamond?

Its dispersion is more than twice diamond's, so it can show more colored fire, depending on cut and lighting. Sparkle is not a reliable way to tell them apart, because cut quality and lighting change it more than the material does.

Is moissanite harder than diamond?

No. Diamond is 10 on the Mohs scale and moissanite is 9¼. Moissanite is still much harder than most gemstones, and GIA rates its toughness as excellent, but it can chip under hard wear.