How to Spot a Fake Diamond: What These 13 Tests Really Mean!

Diamonds and Fake Diamonds

A number of white gems can have a diamond-like appearance. Common diamond simulants include cubic zirconia, white zircon, white topaz, white sapphire, moissanite, white spinel, quartz (rock crystal), and glass. However, these gems all have different physical, chemical, and optical properties.

These 13 procedures will test the properties of your gem to help you determine what it is.

However, the best way to spot a fake diamond is to take it to a jeweler for thermal and electrical conductivity tests. This requires an expensive machine, but it can tell the difference between diamond and moissanite.

Note that lab-created diamonds have identical properties to mined diamonds and will pass all of these tests. The only way to determine whether a diamond is lab-made or mined is to send it to a gemological laboratory.

How to Spot a Fake Diamond While Shopping

If you're shopping for a diamond, you won't be able to perform all these tests. Instead, be sure to buy from a reputable dealer with a good return policy. If the gem doesn't have a grade report from a gem laboratory, ask if they'll send it to the lab for you. For a real diamond and a reliable seller, this shouldn't be an issue.

If you're not sure where to buy, try trusted online dealers like James Allen and Blue Nile. They provide magnified, 360° videos of their diamonds for you to see their quality. Better yet, their diamonds come with grade reports!

Test #1: Thermal Test (The Best Test to Spot a Fake Diamond!)

In this video, FAKE and REAL shows you how to use a diamond detector! Video licensed under Creative Commons.

What You'll Need

A diamond tester (about $15 online).

How to Check for Thermal Conductivity

Diamond testers measure thermal conductivity in a gemstone. Each test takes about 30 seconds. Make sure your gem is clean and follow the instructions for your diamond tester.

What do the Test Results Mean?

A diamond tester will only test positive for diamond and moissanite. Synthetic moissanite has been used as a gemstone only since the 1990s, so if your piece is from an earlier era, it's definitely a diamond if it passes this test!

Test #2: Facet Doubling

What You'll Need

A jeweler's loupe or magnifying glass and some patience.

How to Check for Facet Doubling

For this test, take the loupe and examine the stone closely. If you've never used a loupe before, hold it in front of your eye and bring the stone closer until it's in focus. You should be able to see the back facets through the table (top) of the gem.

Look for doubled or fuzzy facets through the stone. Check multiple angles through the crown facets.

What do the Test Results Mean?

Diamonds are singly refractive, but many imitations are doubly refractive. At certain angles, you'll see doubled back facets in doubly refractive gems. While this effect often appears obvious in zircon, other diamond substitutes show this much more subtly.

If the facets are always single and appear crisp, it could be diamond, cubic zirconia, white spinel, glass, gadolinium gallium garnet (GGG), or yttrium aluminum garnet (YAG).

However, if you've seen facet doubling, the gem might be moissanite, white zircon, white topaz, quartz, or white sapphire.

Test #3: UV Fluorescence

What You'll Need

A blacklight.

How to Check for Fluorescence

In a dark room, shine the blacklight on your gem and see if it fluoresces.

What do the Test Results Mean?

If your gem glows blue under the blacklight, it's probably a diamond. However, if it glows another color, you most likely have an imitation. That said, some diamonds fluoresce yellow.

Gems that didn't react to the ultraviolet light could be either diamond or an imitation.

Test #4: Newspaper/Dot Test

What You'll Need

A newspaper or a piece of paper with a small dot drawn on it. This test is only appropriate for loose gems, not set ones.

How to Perform the Test

Place the loose gem face-down on newspaper text or a piece of paper with a small dot drawn on it. Can you still see the shape of the letters or the dot?

What do the Test Results Mean?

If you can still see the shape of the letters, even if they're wavy or distorted, you definitely don't have a diamond. Since diamonds bend light so intensely, the text or dot will be entirely unrecognizable. In fact, with the dot test, you may not see the dot at all!

Test #5: The Fog Test

What You'll Need

Just yourself!

How to Perform the Test

Before you start, make sure your gem is clean. Dirt and oil can affect your results.

Breathe on the gem and quickly take a look at it. Does it fog up? How long does it stay fogged up?

What do the Results Mean?

This procedure tests thermal conductivity, like a diamond tester. Diamond and moissanite shouldn't fog up for more than a couple seconds, but other imitations will!

However, this test isn't perfect. First, there's no set amount of time that a diamond will fog up. Ambient temperature and humidity could also change the results of the test. If you can, try comparing a stone that you know is diamond to the one you're testing.

Test #6: The Water Test

What You'll Need

A glass of water and a loose gem.

How to Test a Gem With Water

Drop the loose stone into the water and see how quickly it sinks to the bottom.

What do the Results Mean?

Diamonds are dense and will sink quickly, while certain imitations will sink more slowly. If your gem doesn't immediately sink to the bottom, it's likely a glass or quartz imitation. However, other imitations, including cubic zirconia, will also sink quickly.

Test #7: The Sparkle Test

What You'll Need

Your eyes — and some experience with these gems.

How to Look at Sparkle

Closely examine the gem's sparkle under a bright light. Note the white light and colored light coming from the gem. Also, note the contrast of light and dark areas.

What do the Results Mean?

Real diamonds sparkle differently from fakes, but without experience it's difficult to recognize the differences. A well-cut diamond will have a good mix of white light, called brilliance, and flashes of color, called dispersion. Diamonds also display a strong contrast of light and dark areas.

If you're not familiar with these nuances and have no known diamonds to compare with test stones, this isn't the best way to spot a fake diamond.

Test #8: Facet Edges

What You'll Need

A jeweler's loupe or magnifying glass.

How to Examine Facet Edges

Using a loupe or magnifying glass, look at the edges of the gem where the facets meet. Are they rounded, chipped, or abraded?

What do the Results Mean?

If the facet edges of your gem are rounded, it's certainly not a diamond.  However, most imitations can have sharp facet edges.

In spite of their fame as the hardest material on earth, diamonds aren't indestructible. They can chip, and diamond can still scratch diamond. So, if you've found a chip on the gem, it could still be a diamond. Nevertheless, a highly abraded surface indicates a softer material.

Test #9: Weighing and Measuring

What You'll Need

A high-accuracy gem scale, calipers (you can use a ruler, but this will be less accurate), and a loose gem.

How to Test for Density

Measure your gem across its widest point. Then, place the loose gem on the scale. Compare the weight in carats and the width to this chart of standard gem sizes.

What do the Results Mean?

Unless your stone is cut unusually shallow or deep, the weight and size should match up with the chart. If you used a ruler, keep in mind that there's some uncertainty in your measurement.

Spinel, topaz, and moissanite have similar densities to diamond, so they could test within the right range. Because other imitations are heavier or lighter, their weights and measurements won't match up with those in the chart.

Test #10: Jewelry Metal Markings

What You'll Need

A loupe or magnifying glass (or very good eyes!)

Where to Look for Markings

If you have a set gem, you're in luck! Markings on the jewelry metal can give you clues to what the gem might be. Look closely at the backs of settings or the inside of the ring band and note any markings.

What do the Markings Mean?

If it's set in white gold, yellow gold, rose gold, or platinum, it could be a diamond. However, a gem set in silver is usually an imitation. That said, many moissanites and white sapphires are also set in gold or platinum for modern engagement rings.

Jewelers mark gold using karats, so "10K" "14K" and "18K" indicate a gold setting. "PT" and "Plat" signify platinum. "STR" and "Ster" mean sterling silver.

Three-digit numbers also indicate purity. Gold may be marked with "585" or "750" while "900" or "950" designates platinum. "925" means sterling silver.

"CZ" indicates that the gem is cubic zirconia.

Test #11: Clarity Imperfections

What You'll Need

A jeweler's loupe and a lot of experience. This test is best left to the experts, but you may be able to spot something.

How to Check Clarity

Make sure the gem is clean before you start. (You don't want to mistake a spot of dirt for an imperfection). Look for imperfections in your gem, such as dark or light spots or tiny crystals in the gem. Take note if there's anything that looks like a bubble.

What do the Results Mean?

If you've found bubbles, it's certainly an imitation. The processes to create cubic zirconia and glass often create bubbles within the structure.

If you've identified imperfections, it could be a diamond, but it could also be another stone.

If you haven't found anything, it could be either a diamond or an imitation.

Test #12: Scratch Test

What You'll Need

Ideally, a full gemstone hardness kit. Otherwise, some material of known hardness, as high as possible.

How to Test for Hardness

WARNING! This is a destructive test that will damage the gem if it's not a diamond. If you'd like to keep the gem looking nice, don't perform this test.

Using the gem, try to make a small scratch on the material. Look closely for a scratch mark.

What do the Results Mean?

If the material isn't scratched, you don't have diamond, but something of a lesser hardness than the material you tested. For example, if you tested your gem on a quartz crystal and did not scratch quartz, the gem would likely be glass. However, if it scratches quartz it could be almost any of the common imitations.

A piece of low-grade corundum (sapphire) would tell you more. If the gem scratches corundum, it's diamond, moissanite, or white sapphire.

Test #13: Shatter Test

What You'll Need

A glass of water, a lighter or candle, and pliers. Safety glasses recommended.

How to Test for Heat Sensitivity

WARNING! This is a destructive test that can ruin the gem if it's an imitation as well as damage the jewelry setting. Don't perform this test if you'd like to keep the gem.

Hold the gem in pliers over the flame and heat the gem for about 40 seconds. Then drop the gem into the water.

What do the Results Mean?

If the gem shatters, it's certainly not a diamond. (It was probably cubic zirconia or glass). However, the test doesn't prove that a surviving gem is a diamond. Moissanite, white sapphire, white topaz, white spinel, YAG, and GGG all have low or no heat sensitivity and may survive this test.

This test can tell you one more thing. Since moissanites temporarily change color when they're heated, if the gem turned brown, then maybe green or yellow, it's a moissanite.

What's the Best Way to Spot a Fake Diamond?

You've probably noticed that none of these tests will 100% prove that your gem is a diamond.  The only way to be sure is to take it to a jeweler or send it to a gem lab.