How to Spot Fake Diamonds: Home Checks and Real Tests

A "fake diamond" is usually one of three things: cubic zirconia (CZ), moissanite, or glass. All three can look bright in a store light. What they cannot do is copy diamond's physical properties exactly, and that is what testing measures.

Property	Diamond	Moissanite	Cubic zirconia
Mohs hardness	10	9.25	8 to 8.5
Refractive index	About 2.42	About 2.65 to 2.69	About 2.15 to 2.18
Fire (rainbow flashes)	Strong	Much stronger, often obvious	Strong, slightly glassy
Refraction	Single	Double (facet edges look doubled)	Single
Heat conductivity	Very high	High (fools thermal testers)	Low
Weight for the same size	Baseline	Slightly lighter	About 1.7 times heavier

Lab-grown diamonds are not on this list because they are not fakes. They are real diamond and test as diamond. See are lab diamonds real for why.

These checks are free and can rule out cheap glass or CZ. None of them proves a stone is diamond.

Home check	How it works	What it can tell you	Where it fails
Fog test	Breathe on the stone; diamond clears in a second or two	Glass and CZ often stay fogged longer	Moissanite clears fast too; dirt and oil change results
Read-through test	Place a loose stone face down on print	If you can read clearly, it is likely not diamond	Only for loose stones; cut quality changes it
Loupe look	10x magnifier at the facet edges	Rounded, chipped edges point to softer stones; doubled lines point to moissanite	Takes practice; tiny pavé stones are hard to see
Rainbow check	Look at the stone in sunlight	Heavy rainbow fire on every flash suggests moissanite	Diamonds show fire too; lighting changes everything
Price and paperwork	Ask what the stone is in writing	A clear written answer is the best home check there is	Only as good as the seller

Skip the "scratch the glass" and "drop it in water" tricks. They prove little and can chip a real stone or damage a setting.

A jeweler's 10x loupe costs little and shows a lot once you know where to look. Hold it close to your eye, bring the stone up until it is sharp, and check these points:

Where to look	Diamond	Look-alike
Facet edges	Sharp, crisp lines even after years of wear	CZ and glass: rounded, nicked or scratched edges
Through the side of the stone	Single, sharp back facets	Moissanite: back facet edges look doubled
Inside the stone	Usually tiny natural crystals or feathers, even in VS grades	CZ: often perfectly clean, sometimes tiny gas bubbles
The girdle (outer edge)	Frosted, polished or faceted; may carry a laser inscription	Glass: mold lines or a smooth, waxy look
The setting	Prongs or beads holding each stone firmly	Glued stones, uneven heights, empty spots

A loupe will not settle every case, but it rules out a lot of glass and worn CZ in seconds.

The pen-style diamond tester sold online measures how fast the stone carries heat. Diamond carries heat very fast, and so does moissanite. That is why a basic thermal tester will beep "diamond" on moissanite.

A multi-tester adds an electrical conductivity reading. Most moissanite conducts a small current; most diamonds do not, so the multi-tester separates them. There is one known exception: rare type IIb diamonds, including some blue stones, also conduct electricity and can read as moissanite. When a result is unclear, a gem lab settles it.

• Thermal only: separates diamond and moissanite from CZ and glass. Does not separate diamond from moissanite.
• Multi-tester (thermal plus electrical): separates diamond from moissanite in almost all cases.
• Lab instruments: needed to tell lab-grown from natural. Neither handheld tester does this.

Moissanite is the most common stone sold as "diamond" by dishonest sellers, because it is hard, bright and passes the cheap tester. Clues: very strong rainbow fire, a slightly different sparkle pattern, and doubled facet lines when you look through the side of the stone with a loupe. Moissanite is a legitimate gem when it is sold as moissanite. The problem is only when the label says diamond.

The same goes for "VVS" listings. VVS is a clarity grade for diamonds, not a stone. A listing that says "VVS moissanite" is using a diamond word to sell a different gem; read what VVS diamonds are before trusting the term.

CZ is the stone in most cheap iced-out chains, pendants and earrings. It is softer than diamond, so after months of wear the facet edges round off and the surface picks up scratches and a cloudy film. Diamonds keep sharp edges for decades. CZ is also much heavier than diamond for the same size, which a jeweler can measure on a loose stone.

Glass is softer still and usually looks dull next to any real gem. It fails the fog test and the loupe test quickly.

Iced chains, pendants and watches are covered in small stones called melee, which makes testing harder. A tester probe has to touch each stone, and one real stone does not prove the rest are. Read melee and pavé for how these pieces are built.

• Test several stones in different areas, not just the big center ones.
• Look at the metal: a piece sold as gold should carry a karat stamp, and green or worn-through spots point to plating.
• Ask for total carat weight, stone type and metal karat on the receipt.
• For watches, the dial, bezel and bracelet can each have different stones; how to check diamonds on a watch covers that.

The longer version for chains and pendants is in the iced out jewelry buying guide.

Many fakes can be spotted before the piece is in your hand. The listing gives them away:

Red flag	Why it matters
"10 ct VVS diamond chain" for a few hundred dollars	Even lab-grown diamonds at 10 carats cost thousands. Our Round Lab Diamond Necklace 9.18 ct is $5,300.
"Passes diamond tester" as the main selling point	That is the standard pitch for moissanite, which passes basic testers.
"Simulated diamond", "diamond alternative", "lab stone"	These are not diamonds. "Lab-grown diamond" and "lab stone" mean very different things.
No metal karat stated, or "gold tone"	Usually plated brass or steel under the stones.
Seller will not write the stone type on the receipt	A real diamond seller has no reason to refuse.

The surest protection is paperwork. A grading report from GIA or IGI identifies the stone, its 4Cs and whether it is natural or lab-grown, and the report number can be checked online. For small stones there is usually no report, so the seller's written description matters: "lab-grown diamond" or "natural diamond", total carat weight, and gold karat.

The FTC Jewelry Guides require that a lab-grown diamond be clearly disclosed as lab-grown, and that simulants like CZ and moissanite not be described as diamond. A seller who will not put the stone type in writing is telling you something.

A tester that says "diamond" on a lab-grown stone is working correctly. Lab-grown diamonds have the same carbon crystal as mined ones, so they pass every handheld diamond test. The question for a buyer is not real or fake, but natural or lab-grown, and whether the price matches what you were told. A lab-grown stone sold as natural is a fraud; the same stone sold as lab-grown is a fair purchase.

A handheld tester answers one question: diamond or not. It cannot tell a lab-grown diamond from a natural one, because both are diamond. That second question matters for price, and only lab instruments answer it, by looking at growth patterns, fluorescence and trace elements.

• Origin: natural or laboratory-grown, stated on the report.
• Grades: carat weight, color, clarity and, for rounds, cut.
• Inscription: many graded stones carry a tiny laser-inscribed report number on the girdle, visible with magnification.
• Verification: GIA and IGI let you check a report number online and compare it with the stone.

Inscriptions can be forged. GIA has documented moissanite carrying fake diamond inscriptions, so match the report's measurements and weight to the stone, not just the number.

Any established jeweler with a multi-tester and a loupe can screen a stone in minutes. For a definitive answer on a large stone, or to learn whether it is natural or lab-grown, send it to a gem lab for a report.

If you want to see what real diamonds look like before you test anything, every house piece is listed with its stone origin: lab-grown or natural. Compare a Mixed Natural Diamond Pendant, 10K Two-Tone Gold ($9,500) with a Round Lab Diamond Necklace 18.48 ct, 14K Yellow Gold ($6,500) at the showroom, 7900 NW 27th Ave, Unit 506, Miami. Our full range is in diamond jewelry.

• "Real diamonds cannot be scratched." Diamond is the hardest natural material, but it can chip on a hard knock. Scratching a stone to test it can damage it.
• "A diamond will not sparkle in the dark." Sparkle needs light; every stone looks dull without it.
• "Lab diamonds are fake." They are real diamonds with a different origin, and they pass diamond testers.
• "A certificate means it is real." Only if the report matches the stone. Check the number, the measurements and the weight.