How to Tell if a Diamond is Real: 10 Tests

How to Tell if a Diamond is Real: 10 Tests

Finding out if a diamond is real is not as straightforward as many think. High-quality fakes such as moissanite and cubic zirconia, and synthetic lab-created diamonds, pass some basic tests that catch glass or low-end stones. If you care about accuracy, it helps to use more than one method and know each one's flaws. Here are 10 tests, clear instructions, facts to know, and the limits of each.

1. The Water Test

Drop the loose stone into a clear glass of water. A real diamond will sink because it has a density of about 3.51 grams per cubic centimeter. Many fakes, like glass, will float or bob near the top.  

Points to watch:

• Cubic zirconia weighs even more (about 5.6 to 6.0 grams per cubic centimeter). It will also sink, possibly faster than a diamond.
• Moissanite is lighter (about 3.22 grams per cubic centimeter) but still sinks.
• This test weeds out only low-density fakes. It cannot tell cubic zirconia or moissanite from true diamonds.
• You need the stone loose. It does not work with mounted jewelry.

2. The Fog Test

Hold the stone with tweezers and exhale on it. Real diamonds disperse heat quickly, so any fog disappears almost at once. Most artificial stones will keep a foggy look for a couple of seconds or more.

Tips:

• Moissanite dissipates heat even more quickly than diamond, so it clears also.
• Fakes like glass or plastic will retain fog for two to four seconds on average.
• A real diamond, in a 22°C (72°F) room, will clear in less than one second.
• Do this on a clean, dry stone. Oil and dirt throw off the test.

3. The UV Light Test

Use a simple portable UV flashlight in a dark room. Shine the light onto the stone and watch for a glow. About 25 to 35 percent of natural diamonds show blue fluorescence, but many true stones do not react. Synthetic stones, especially cubic zirconia and glass, usually show little or no reaction under UV.

Recent Data (2024-2025):

• According to GIA, a third of natural diamonds test blue, while most fakes and synthetics show nothing.
• A small number of cubic zirconia can glow faint yellow or white.
• Moissanite is less likely to show blue.
• No reaction does not mean a fake.

4. The Dot (or Newspaper) Test

Place the flat side of the loose stone on a piece of paper with a small, dark dot drawn on it. Try to look through the stone from the top. With a real diamond, you will not be able to see the dot. The way diamond bends light hides the mark or at best makes it impossible to see clearly.

Test Limits:

• Cubic zirconia and glass may let you see the dot or a blurry shape.
• Moissanite's high refractive index means it can also hide the dot, so it passes the test.
• This only works on loose, unmounted stones.

5. The Sparkle Test

Hold the diamond under bright white light. Real diamonds show a mix of strong white light (called brilliance) and some rainbow flashes (fire). Cubic zirconia has more colorful, scattered rainbow flashes, which look more like glare than sparkle.

Tech-Supported Sparkle:

• Moissanite creates even more rainbow flashes than diamond (dispersion of 0.104 versus 0.044 for diamond).
• Cubic zirconia also out-sparkles natural stones but with less sharpness.
• In 2025, apps like GemScan AI claim to use a phone camera to recognize diamond sparkle patterns. These have about 90 percent accuracy in basic tests, but nothing beats the eye.
• Comparison side-by-side is easiest with both pieces unmounted.

6. The Weight Test

Use a sensitive jewelry scale (accurate to 0.01 grams). Diamonds are lighter than cubic zirconia, though heavier than moissanite. A one-carat diamond weighs 0.2 grams. Cubic zirconia comes in around 60-70 percent heavier for the same size, while moissanite is about 10 percent lighter than diamond.

Practical Example:

• For a 6.5mm round brilliant: diamond (1 carat, 0.2g); cubic zirconia (1 carat equivalent size, up to 0.32–0.35g); moissanite (1 carat equivalent size, about 0.18g).
• Mounted stones cannot be tested this way.

7. The Heat Resistance Test

Genuine diamonds can handle heat that would ruin most fake stones. You can heat the stone with a lighter for about 30 seconds and then drop it in cold water. Diamonds will survive; glass and many fakes will shatter due to rapid expansion and contraction.

Warnings:

• Moissanite can also survive this test, as it is highly heat resistant.
• You risk damaging other parts of a ring or setting.
• Do not do this on heirlooms or valuable jewelry.

8. The Magnifying Glass or Loupe Test

A standard jeweler's loupe (10x) lets you examine the stone's features and flaws. Natural diamonds often have small internal marks or blemishes. Cubic zirconia looks flawless under 10x. Moissanite shows a telltale double image of the back facets, called double refraction, when looked at from certain angles.

Things to Find:

• Laser engravings on the girdle (edge) can show if a stone is lab-grown or certified.
• Moissanite sometimes shows two back facet edges at once (doubling).
• Cubic zirconia looks too perfect and may show bumpy or grainy polish.

9. The Diamond Tester (Thermal Conductivity Test)

A diamond tester measures how quickly heat passes through the stone. Diamond and moissanite conduct heat very well. Cubic zirconia and glass do not.

Limitations:

• Moissanite may "pass" as diamond on older or cheap testers.
• Dual testers, which test both heat and electrical conductivity, can tell moissanite from diamond but cost more money. In 2025, new models claim up to 98 percent accuracy and start at $500.
• Lab diamonds have the same heat and electrical properties as mined diamonds, so testers cannot tell them apart.

10. The Fluorescence Test

Under a stronger UV lamp, some diamonds glow blue, but others do not. Fluorescence is graded by labs and can affect how the diamond appears in certain lights. About 65 to 75 percent of diamonds do not fluoresce at all.

Market Data (2024-2025):

• Diamonds graded D to F that show strong blue fluorescence often sell for 7–15 percent less.
• Stones in the I to M color range with some fluorescence may sell at a slight premium.
• Fluorescence is not a sure way to spot a fake, but adds to the overall picture.

What Each Test Misses

Each method points out certain fakes, but none is perfect alone:

Water, fog, and dot tests are not helpful with moissanite.

Sparkle, weight, and heat tests will not separate moissanite from diamond for most people.

UV light and fluorescence tests fail about two-thirds of the time because most real diamonds do not fluoresce, and some fakes can mimic the effect.

Diamond testers often mistake moissanite for diamond, unless you have the newer version with both heat and electrical probes.

Lab-grown diamonds pass all tests above except for inscriptions or lab equipment.

Lab-Grown, Market, and 2025 Context

Lab-grown diamonds now make up nearly 40 percent of new diamond sales by weight, based on an IGI report from early 2025. They have the same chemical, physical, and optical properties as mined diamonds. They carry inscriptions like "LG" or "LabGrown" on the girdle. These are best checked under strong magnification.

Cubic zirconia has become harder to spot, with some stones cut to mimic the exact diamond measurements and finished with surface treatments. However, density and dispersion still set them apart.

Moissanite is the closest visual faker. It is rare in nature but widely grown in labs for jewelry. Look for double refraction and excessive fire to pick it out from diamond.

Scams and What to Watch (up to 2025)

Fake certificates: Some sellers etch fake serial numbers onto stones. GIA and IGI both have public online databases to check that numbers match their real registry.

Social media trading: Facebook Marketplace and similar have a fraud rate of over 25 percent for "diamonds," most not passing basic home tests.

Resale apps have seen a surge in synthetic stones sold as mined over the last year.

Tips for Buyers

Run at least three tests before assuming a diamond is genuine.

Do not use destructive tests on jewelry you value.

Use UV, sparkle, and magnification tests for mounted stones.

Only buy stones with certificates from labs you trust (such as GIA or IGI).

Cross-check any laser engraving with the issuing lab's database.

Get an independent appraisal for any stone worth more than $500.

Be skeptical of perfect clarity at low prices, real natural diamonds have flaws.

What the Experts Say

In 2024, Jeweler's Mutual stated that a third of the insurance claims it received for diamonds involved a fake or misrepresented stone. The Gemological Institute of America continues to warn consumers against relying on single, home-based tests. Lab equipment, especially in 2025, can fully identify lab-grown stones, fakes, and treatments, but costs for consumers are usually over $100 per stone.

The safest way to confirm a diamond's authenticity is still to have the stone examined by a certified gemologist with advanced lab tools. Home testing is helpful for weeding out obvious fakes and poor-quality imitations, but it will not catch the high-end synthetics or moissanite.

Conclusion

You do not need a lab or expensive tool to catch most fake diamonds, but knowing several methods and accepting their limits makes a big difference. Water, fog, sparkle, and magnification tests help rule out the cheaper fakes. To separate moissanite and lab diamonds from real, mined stones, you need more advanced tools and certifications. For anything valuable, or if a deal seems suspiciously good, it pays to get a professional opinion. The trend for 2025 is clear: more fakes, more advanced lab-grown stones, and more tools to spot them, if you know what to use and where each method falls short.