How to Tell if a Diamond Is Real

Which home tests for a real diamond mean anything

Every home test rests on one property of diamond, and every one of those properties is shared by at least one stone that is not diamond. That is why so few of these tests decide anything.

The fog test

Breathe on the stone as you would on a mirror. A diamond moves heat away quickly, so the fog is meant to clear in a moment, while glass and cubic zirconia hold it a little longer. The idea is sound, but the result depends on how clean the stone is, how warm your breath is and how damp the room is.

Moissanite clears as fast as diamond, and so does a lab-grown diamond. The fog test can catch a crude fake. It cannot confirm a real diamond.

The water test

The claim is that a real diamond sinks in a glass of water. Almost every stone sinks, including cubic zirconia, moissanite, sapphire and glass, so the result says nothing. The test also needs a loose stone, because a set diamond cannot be dropped in. Treat this one as folklore.

The newspaper test

Lay a loose stone flat side down over printed text. A well-cut round diamond bends light so sharply that the print cannot be read through it. This is the most reasoned of the home tests, and it has a use.

If you can read the words, the stone is very likely glass or a weak imitation. The limits are plain, though. Moissanite also blocks the print, fancy cuts behave differently from round ones, and the test only works on a stone out of its setting.

The black light test

Some diamonds glow blue under ultraviolet light and many do not glow at all. Some imitations glow too. Fluorescence is a characteristic that a grading report records, not a pass or fail sign of identity. A stone that stays dark under a black light has told you nothing, and neither has one that glows.

The scratch test and the heat test

The scratch test says a diamond will scratch glass. It will, but so will moissanite, sapphire and cubic zirconia, and you risk chipping the stone in the attempt. Hardness is not toughness. A diamond resists scratches, but a hard knock or a scrape against something rigid can still damage a facet or loosen the stone in its setting.

The heat test means holding a flame to the stone and dropping it into cold water. Diamond tolerates the heat, but the setting may not, and moissanite survives it too. Do neither.

How to tell if diamonds are real with a loupe

A jeweler’s loupe is a small folding magnifier. For anyone working out how to tell if diamonds are real, what it shows is worth more than any kitchen test. It works on a set stone as well as a loose one. Read the signs together, because none of them is a verdict alone.

• Sharp facet edges and flat faces point to diamond, which is hard enough to hold them. Cubic zirconia and glass are softer, and their edges look rounded and worn.
• Natural diamonds usually carry tiny internal features such as small crystals or fine cracks. A stone with none is not a fake, since some diamonds are very clean, but round bubbles inside point to glass.
• Look down through the top of the stone toward the back facet edges. In moissanite they appear doubled, because moissanite splits light in two. Diamond does not.
• A diamond returns mostly white light with restrained flashes of color. Cubic zirconia and moissanite throw more rainbow color, and a stone that looks like a disco ball is more likely a simulant.
• Many graded diamonds carry a laser inscription of the report number on the girdle, the thin edge around the stone. A loupe can read it. Its absence proves nothing, because many diamonds have none.

Weight helps when the stone is loose. Cubic zirconia is noticeably heavier than a diamond of the same size, and moissanite is a little lighter. For a definite answer you need a tester or a report.

Fake diamond names and how each one differs from diamond

A stone made to look like diamond is called a diamond simulant, and each simulant has a name of its own. Every fake diamond name below belongs to a different material, and the plain differences that separate each one from diamond are set out under it.

Cubic zirconia

Cubic zirconia, often shortened to CZ, is a laboratory-made zirconium oxide and the most widely used imitation. It weighs noticeably more than diamond at the same size, throws too much color, and is soft enough that its facet edges round off and its surface clouds with wear. It moves heat slowly, so a thermal tester reads it as not diamond.

Moissanite

Moissanite is silicon carbide. Nearly all of it sold today is made in a laboratory, and it is a genuine gemstone in its own right, but it is not diamond. It is very hard and brilliant, gives off more rainbow color than diamond, and shows doubled facet edges under a loupe. Its heat conduction is close enough to diamond that a thermal-only tester can read it as diamond, which is why combination testers exist.

White sapphire

White sapphire is colorless corundum, the same mineral family as blue sapphire and ruby. It is hard and durable, but it has far less brilliance than diamond, so it looks glassy and quiet beside one. Light passes through it without the same sparkle.

White topaz

White topaz is colorless topaz. It is softer than diamond and sapphire, it can chip along its cleavage, and its sparkle is muted. It is a real gemstone sold on its own terms and becomes a fake only when passed off as diamond.

Glass

Glass, sold as crystal, paste or rhinestone, is the softest of the group and the easiest to identify. It scratches easily, often shows round bubbles under a loupe and lacks the brilliance of diamond. A few trade names borrow the word diamond without being diamond. A Herkimer diamond, for one, is a quartz crystal.

Lab-grown diamond is not on this list

A lab-grown diamond has the same crystal structure, composition and thermal conductivity as a mined diamond, so it is real diamond that happens to have been grown in a laboratory. If a piece is described as lab-grown diamond, it is diamond. If it is described as lab-created moissanite or lab-created white sapphire, it is not.

What a thermal and electrical tester reads

A thermal tester touches a metal probe to the stone and measures how fast heat moves away. Diamond moves it faster than any other common gemstone. Cubic zirconia, glass and most other simulants move it slowly, so a thermal tester separates them at once. Moissanite moves heat almost as well as diamond, so an older thermal-only tester can read it as diamond.

A combination tester adds an electrical reading. Moissanite conducts electricity and almost all diamond does not, which lets the tester tell the two apart. A few rare blue diamonds do conduct, so a person reads the result, not a light on the handset.

The tester answers what the stone is made of. It does not answer where the stone was grown.

Lab-grown diamond has the same thermal conductivity as mined diamond, so it passes a thermal tester and a combination tester alike. No handheld tester separates lab-grown from natural. That takes laboratory equipment, and the answer is what a grading report records. The full reasoning sits in our guide to whether lab-grown diamonds pass a tester.

The only answer that settles it

A grading report from an independent laboratory such as GIA or IGI identifies the stone, states whether it is natural or lab-grown, and records its cut, color, clarity and carat weight. When the report number is laser-inscribed on the girdle, the paper can be matched to the stone under a loupe. If you are buying, ask for the report and read it against the stone. Our diamond buying guide explains what each grade on a report means.

If you already own the piece and have no paperwork, a jeweler with a loupe and a combination tester can tell you what the material is. Only a laboratory can say whether a diamond is natural or lab-grown. To see how a stone is described when it is sold as diamond, browse our diamond jewelry. Both natural loose diamonds and loose lab grown diamonds are real diamonds, and each stone's page says which it is.