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Education 9 min read

How to Tell if a Diamond Is Real (and if It's Lab-Grown)

By Yareli, Founder and Designer

Tests That Help, Tests to Skip and the Proof That Settles It

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Breathing on a stone, looking through it and running a diamond tester over it can each tell you something, but none of them can prove a diamond is real, and none can tell a lab-grown diamond from a natural one. Here is what every common test can and can't show, how look-alikes such as moissanite and cubic zirconia give themselves away, and how a grading report and its laser inscription settle the question.

The experts you can trust: Mine & Grown guides are written with jewelry experts and reviewed for accuracy. Questions about a specific piece? Our team is happy to help.

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The Short Answer

Home tests can expose an obvious fake, but they cannot prove a diamond is real. A fog test, a loupe or a basic diamond tester will catch glass and cubic zirconia most of the time. Moissanite passes several of them, and a well-made setting can hide the clues the rest rely on.

A lab-grown diamond is a real diamond. It is the same crystal of carbon as a mined one, so it passes every home test and every standard diamond tester. Telling lab-grown from natural takes laboratory instruments.

The proof is a grading report from an independent laboratory such as IGI or GIA, matched to the stone by the report number laser-inscribed on its girdle, the thin edge around the widest part of the diamond. If the stone has no report, a gemologist with testing equipment can identify it, or send it to a lab.

Diamond Tests at a Glance

TestWhat it can showCan it prove a diamond is real?Can it tell lab-grown from natural?
Fog test (breathe on it)A diamond clears in a second or two; glass and cubic zirconia stay misty longerNo: moissanite clears just as fastNo
Read-through test (loose round stones)Print is hard to read through a well-cut round diamondNo: moissanite blocks print tooNo
10x loupeCrisp facet edges; doubled facet lines point to moissaniteNoNo
Weight on a carat scale (loose stones)Cubic zirconia weighs about 1.6 to 1.7 times as much as a diamond of the same sizeNoNo
UV lightMany diamonds glow blue, many don'tNoNo
Thermal diamond testerDiamond against glass, cubic zirconia and sapphireNo: moissanite also reads as diamondNo
Multi-tester (heat and electricity)Also separates moissaniteNearly: a few lab-grown diamonds read as moissaniteNo
Grading report and laser inscriptionIdentity, grades and origin of that exact stoneYes, when the inscription matches the reportYes
Gemological laboratoryDiamond or not, lab-grown or natural, treatmentsYesYes
Is a Lab-Grown Diamond a Real Diamond?

Is a Lab-Grown Diamond a Real Diamond?

Yes. A lab-grown diamond is pure carbon arranged in the same cubic crystal as a diamond from the earth. It has the same hardness, 10 on the Mohs scale, the same way of bending light (a refractive index of 2.42) and the same fire. The difference is where it formed: in weeks, inside a growth chamber, instead of over billions of years underground.

In 2018 the US Federal Trade Commission removed the word "natural" from its definition of a diamond, so a lab-grown diamond is a diamond in the eyes of US law. Sellers must say clearly that it is lab-grown, laboratory-grown or laboratory-created, and grading reports do the same.

A look-alike, or simulant, is a different material that imitates a diamond: moissanite, cubic zirconia, white sapphire or glass. These are what the tests below are designed to catch. Read What Are Lab Grown Diamonds? and Natural vs Lab Grown Diamonds for how the two kinds of diamond compare.

Home Tests That Can Help

Home Tests That Can Help

These checks are quick and safe. Use them to rule out an obvious fake, not to confirm a diamond.

  • The fog test. Clean the stone, then breathe on it as you would on a mirror. Diamond spreads heat very quickly, so the mist clears in a second or two. On glass or cubic zirconia it usually lingers for several seconds. Moissanite clears as fast as diamond, and grease on any stone slows the result.
  • The loupe test. Look through a 10x jeweler's loupe. A diamond's facet edges are crisp and sharp; softer stones such as glass and cubic zirconia often show rounded or chipped edges after some wear. Small natural-looking inclusions are common in diamonds. Look at an angle through the top: if the back facet lines appear doubled, the stone is probably moissanite, which splits light in two where diamond does not.
  • The read-through test. For a loose round stone only: lay it upside down on a line of print. A well-cut round diamond bends the light so strongly that the letters disappear; through glass or cubic zirconia you can often make out a blur of them. Moissanite also blocks the print, and fancy shapes vary, so it cannot settle anything on its own.
  • The sparkle check. A diamond returns bright white light with flashes of color. Moissanite throws more than twice as much rainbow color, and cubic zirconia also shows more color than diamond, which can make both look busier under a spotlight. This takes practice and is easy to get wrong.
  • Weight, for loose stones. On a carat scale, cubic zirconia weighs about 1.6 to 1.7 times as much as a diamond of the same size, and moissanite about 10 percent less.
A diamond engagement ring on a marble counter beside a glass of water and a lighter

Home Tests to Skip

  • The water test. Dropping a stone in water tells you nothing: diamond sinks, and so do moissanite, cubic zirconia, sapphire and most glass.
  • The scratch test. Diamond does scratch glass, but so do moissanite, sapphire and cubic zirconia. Diamond is hard but not unbreakable: a sharp knock along its grain can chip it, so rubbing it against glass or sandpaper risks damage for no answer.
  • The heat test. Heating a stone with a lighter and dropping it into cold water can crack a diamond with internal fractures, crack other stones and damage the metal or any smaller stones in the setting.
  • The bite test. Biting proves nothing and can chip a tooth or the stone.

If a stone has passed the safe checks and you still need an answer, the next step is a tester or a professional, not a harder home test. Our guide How to Take Care of Your Diamond Jewelry covers what diamonds can and can't stand.

What a Diamond Tester Can and Can't Tell

What a Diamond Tester Can and Can't Tell

Most handheld diamond testers measure how fast heat flows through the stone. Diamond conducts heat better than any other gem, so a thermal tester separates it cleanly from glass, cubic zirconia and white sapphire.

Moissanite conducts heat almost as well as diamond, so it reads as diamond on a thermal tester. Multi-testers add a second check, electrical conductivity: moissanite carries a small current and most diamonds do not. Even this has an exception. Some lab-grown diamonds, mostly HPHT-grown stones with a trace of boron, also conduct electricity and can be flagged as moissanite by mistake.

No handheld diamond tester can tell a lab-grown diamond from a natural one, because both are diamond. Jewelers use separate screening devices that read how a stone reacts to ultraviolet light, and they send any stone the device flags to a laboratory. Our guide CVD vs HPHT Lab-Grown Diamonds explains the two ways lab-grown diamonds are made.

A jeweler examining a diamond engagement ring through an eye loupe

How to Tell a Lab-Grown Diamond From a Natural One

You can't by eye, and neither can an experienced jeweler with a loupe. A lab-grown and a natural diamond with the same grades look the same in a ring. Laboratories separate them with instruments: spectroscopy, which reads the trace elements and defects in the crystal, and deep ultraviolet imaging, which shows the pattern the diamond grew in. Natural diamonds grow in an octahedral pattern, HPHT diamonds in a cube-and-octahedron pattern and CVD diamonds in flat layers.

In practice the answer is usually written on the stone. Grading labs laser-inscribe lab-grown diamonds on the girdle: IGI uses its report number with the letters LG in front, and GIA writes "Laboratory-Grown" with its report number. In October 2026 we read 52 IGI reports from our own lab-grown diamond search, and every one of them listed an LG inscription. The inscription is far too small to read with the naked eye, so you need a 10x loupe or a jeweler's microscope to see it.

How to Check a Diamond's Grading Report

How to Check a Diamond's Grading Report

  1. Find the report number. It is printed on the report and, for most graded diamonds, inscribed on the girdle.
  2. Look it up. Enter it in IGI's Verify Your Report tool at igi.org or in GIA's Report Check at gia.edu. The lab's own record should show the same report.
  3. Read the title. A lab-grown diamond comes with a laboratory-grown diamond report; a natural diamond's report does not use those words.
  4. Match the stone. Shape, carat weight and measurements to a hundredth of a millimeter should all match. A jeweler can weigh a loose stone, measure a mounted one and read the inscription for you.

A report proves the stone in front of you only when the inscription and the measurements match it. A report on its own, separated from the diamond, proves nothing about another stone. IGI vs GIA explains how the two labs compare.

Diamond, Lab-Grown Diamond and Look-Alikes Compared

MaterialHardness (Mohs)Brilliance and fire (refractive index / dispersion)Reads as diamond on a thermal tester?
Natural diamond102.42 / 0.044Yes
Lab-grown diamond102.42 / 0.044Yes
Moissanite9.252.65 to 2.69 / 0.104: more than twice the fireYes
Cubic zirconia8 to 8.52.15 to 2.18 / about 0.06No
White sapphire91.76 to 1.77 / 0.018No
GlassAbout 5.5About 1.5 / variesNo
Buying a Diamond You Can Trust

Buying a Diamond You Can Trust

The simplest way to know what you own is to buy a diamond that comes with an independent grading report and an inscription that matches it. If you already own a stone without one, ask an independent appraiser or a graduate gemologist to test it, or have a jeweler send it to IGI or GIA for a report.

Every center diamond in a Mine & Grown engagement ring, lab-grown or natural, comes with an IGI or GIA report, and every stone in our diamond search shows its report so you can read it before you choose. Rings are made to order in our Chicago studio in about 4 to 6 weeks. If you want help reading a report, Yareli, our founder and designer, can go through it with you by video or phone.

Real Diamond FAQ

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How can you tell if a diamond is real at home?

Clean the stone and try the fog test, a 10x loupe and, for loose stones, the read-through test. They can expose glass or cubic zirconia, but none of them proves a diamond: moissanite passes several, and only a grading report with a matching inscription, or a gemologist's test, proves it.

What is the fog test for a diamond?

Clean the stone, then breathe on it as you would on a mirror. A diamond spreads heat very quickly, so the mist clears in a second or two; on glass or cubic zirconia it usually lingers for several seconds. Moissanite clears as fast as diamond, and grease on any stone slows the result, so the fog test can rule out an obvious fake but cannot prove a diamond is real.

Is a lab-grown diamond a real diamond?

Yes. A lab-grown diamond is pure carbon in the same cubic crystal as a diamond from the earth, with the same hardness (10 on the Mohs scale), the same refractive index (2.42) and the same fire. In 2018 the US Federal Trade Commission removed the word "natural" from its definition of a diamond, so a lab-grown diamond is a diamond as long as the seller says clearly that it is lab-grown.

Do lab-grown diamonds pass a diamond tester?

Yes. A lab-grown diamond is real diamond, so it passes thermal diamond testers like a natural one. A few lab-grown diamonds, mostly HPHT stones with a trace of boron, can read as moissanite on a multi-tester that also checks electrical conductivity.

Can a jeweler tell if a diamond is lab-grown?

Not by eye or with a loupe. Jewelers use screening devices and send flagged stones to a laboratory, which identifies lab-grown diamonds with spectroscopy and ultraviolet imaging. Most graded lab-grown diamonds are also inscribed on the girdle, for example LG and the report number on IGI stones.

Does moissanite pass a diamond tester?

It passes thermal diamond testers, because moissanite conducts heat almost as well as diamond. A multi-tester that also checks electrical conductivity separates it. Under a loupe, moissanite usually shows doubled facet lines and more rainbow fire.

Does a real diamond sparkle like a rainbow?

A diamond shows bright white light with flashes of color. A stone that throws strong rainbow color everywhere is more likely moissanite or cubic zirconia, though lighting changes how any stone looks.

Does the water test work for diamonds?

No. A diamond sinks in water, but so do moissanite, cubic zirconia, sapphire and most glass.

Can you tell if a diamond is real by scratching glass?

No. A diamond does scratch glass, but so do moissanite, sapphire and cubic zirconia, so the test proves nothing. Diamond is hard but not unbreakable: a sharp knock along its grain can chip it, so rubbing a stone on glass or sandpaper risks damage for no answer.

How do I check if a diamond report is real?

Look up the report number on the lab's own site, IGI's Verify Your Report or GIA's Report Check, then match the shape, carat weight, measurements and girdle inscription to the stone.

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