How to recognise a real diamond: 7 simple tests

At a gem market in India, a vendor hands me a transparent stone with an intense brilliance. "Natural diamond, madam. The best price in the city." The stone shimmers under the neon lights. Too perfect, too clear, and far too inexpensive. Two minutes with a 10x loupe revealed the deception: a well-cut cubic zirconia, whose refractive index betrayed its synthetic nature.

Distinguishing a real diamond from an imitation, a synthetic diamond, or a substitute stone requires a trained eye and a few simple tests. Some can be done at home; others require professional equipment. We have gathered the 7 most reliable methods for authenticating a diamond, from the most accessible to the most technical.

In this article, you will discover at-home tests (the fog test, the reading test, the loupe test), professional tests (thermal conductivity, specific gravity, UV fluorescence), and the gemological certifications that guarantee authenticity. You will also learn to recognise the main imitations and understand why certain popular methods simply do not work.

Diamond conducts heat 30 times better than cubic zirconia. Breathe on the stone as you would on a mirror. The fog should disappear in under 2 seconds on a real diamond. On an imitation made of glass, cubic zirconia, or moissanite, the fog lingers for 4 to 6 seconds.

This test works because diamond possesses the highest thermal conductivity of all natural materials: 900 to 2,320 W/m·K depending on crystal orientation. Glass peaks at 1 W/m·K, cubic zirconia at 10 W/m·K. The heat from your breath is dissipated almost instantaneously by the crystalline lattice of pure carbon.

Please note: this test cannot distinguish a natural diamond from a synthetic one, as both share the same chemical composition and the same conductivity. It serves as a first filter to eliminate crude imitations (glass, resin, plastic, cubic zirconia), not as a definitive verdict.

Repeat the test 3 times. If the fog consistently disappears in under 2 seconds, you have eliminated 80% of common imitations. Move on to the next test.

Place the Diamond face down on a printed page or section of text. You should not be able to read the letters through the stone. If you can make out any words, even blurred ones, the stone is an imitation.

A Diamond has a refractive index of 2.42, the highest of any common transparent gemstone. Light entering the stone is refracted at such a sharp angle that it exits through the crown as scattered flashes, never in straight lines. The letters become invisible, replaced by a luminous halo.

Glass (index 1.5 to 1.9) and quartz (1.54) allow light to pass through with moderate refraction. You will see the letters distorted but still legible. Cubic zirconia (index 2.15) offers a refractive index close to that of a Diamond, but not strong enough to block the text completely. You will be able to make out letter shapes, slightly blurred.

Moissanite (index 2.65) can fool this test, as its refractive index actually exceeds that of a Diamond. If the stone passes the reading test but fails the fog test, moissanite should be suspected. Proceed to the double refraction test.

A diamond thermal tester measures the thermal conductivity of the stone in real time. You place the metal tip on the table of the diamond, and the device indicates "diamond" or "non-diamond" within 2 seconds. 99% reliability for diamonds versus cubic zirconia and glass.

These testers cost between $50 and $180 depending on their precision. They work by gently heating the tip a few degrees above ambient temperature and measuring the speed at which heat dissipates through the stone. Diamond dissipates heat so rapidly that the sensor registers an almost instantaneous drop in temperature.

Limitation: thermal testers can confuse diamond with moissanite, as moissanite has a thermal conductivity almost as high (120 W/m·K). Some newer models (combined thermal and electrical testers) resolve this issue. Moissanite is an electrical semiconductor, while diamond is an insulator. A combined tester will display "moissanite" if the stone conducts both heat and electricity.

Professional jewellers use these combined testers. If you are purchasing a diamond of more than 0.5 carat outside of a certified channel, investing around $100 in a combined tester is a reasonable precaution.

A Diamond has a density of 3.52 g/cm³. Cubic zirconia, its most common imitation, reaches 5.6 to 6 g/cm³. At an equivalent size, a cubic zirconia weighs 60% more than a diamond.

If you have a precision scale (0.01 g) and know the exact dimensions of the stone, calculate its volume (cone plus cylinder formula for a standard brilliant cut) then its density. A stone measuring 6.5 mm in diameter and 4 mm in depth should weigh approximately 1 carat (0.20 g) if it is a diamond. If it weighs 0.32 g, it is a cubic zirconia.

Moissanite weighs 3.22 g/cm³, very close to diamond. The weight test cannot distinguish it from a diamond. White Sapphire (3.98 to 4.01 g/cm³) and colorless topaz (3.49 to 3.57 g/cm³) have similarly close densities. This test is most effective for identifying cubic zirconia and glass.

Jewellers use a hydrostatic balance to measure density without geometric calculations. The stone is weighed in air, then submerged in water. The ratio of the two weights gives the density. Accuracy of 0.01 g/cm³ is sufficient to distinguish between diamond and cubic zirconia.

The 4Cs (Carat, Color, Clarity, Cut) are the international criteria for diamond grading. They do not alone prove that a diamond is natural, but they can help identify inconsistencies. A seller claiming to offer a 3-carat D FL diamond (flawless, colourless) for $1,200 is being dishonest: such a stone is worth between $95,000 and $145,000.

Carat: the weight of the diamond. 1 carat equals 0.20 grams. A 1-carat diamond measures approximately 6.4 mm in diameter in a brilliant cut. Prices rise sharply at psychological thresholds: a 0.90 ct stone sells for around 30% less than a 1.00 ct stone of equivalent quality.

Color: the GIA scale ranges from D (colourless) to Z (light yellow). D, E, and F are colourless; G, H, I, and J are near-colourless; K, L, and M are slightly tinted. Beyond M, the term "fancy color" applies. A G or H diamond appears white to the naked eye in white gold, and carries a subtle warmth in yellow gold.

Clarity: purity. FL (flawless, with no internal or external inclusions) and IF (internally flawless) are exceptionally rare. VVS1 and VVS2 (very very slightly included) feature inclusions invisible under 10x magnification. VS1 and VS2 (very slightly included) have inclusions visible under a loupe but not to the naked eye. SI1 and SI2 (slightly included) have inclusions visible under a loupe and occasionally to the naked eye for SI2. I1, I2, and I3 (included) have inclusions visible to the naked eye, with a direct impact on brilliance.

Cut: the quality of the cut, graded as Excellent, Very Good, Good, Fair, or Poor. An excellent cut maximises brilliance (reflected white light) and fire (the dispersion of light into colour). A poor cut will render a diamond dull, even when its colour and clarity grades are high. Cut is the only one of the 4Cs shaped by human hands; the other three are entirely natural.

A gemological certificate (GIA, HRD, or IGI) attests to the 4Cs. It does not prove that a diamond is natural (synthetic stones also carry their own certificates, marked "laboratory-grown"), but it confirms that the stated characteristics correspond to the stone. A seller who refuses to provide a certificate for a diamond of more than 0.5 carat should be treated with caution.

The 4Cs (Carat, Color, Clarity, Cut) are the international criteria for diamond grading. They do not alone prove that a diamond is natural, but they help identify inconsistencies. A seller claiming to offer a 3-carat D FL diamond (perfect, colorless) for $1,200 is not being truthful: such a diamond is worth between $95,000 and $145,000.

Carat: the weight of a diamond. 1 carat equals 0.20 grams. A 1-carat diamond measures approximately 6.4 mm in diameter in a brilliant cut. Prices rise sharply at psychological thresholds: a 0.90 ct stone sells for 30% less than a 1.00 ct stone of equal quality.

Color: the GIA scale ranges from D (colorless) to Z (light yellow). D, E, and F are colorless; G, H, I, and J are near-colorless; K, L, and M are slightly tinted. Beyond M, the term "fancy color" applies. A G or H diamond appears white to the naked eye in white gold, and slightly warm in yellow gold.

Clarity: purity. FL (flawless, with no internal or external inclusions) and IF (internally flawless) are extremely rare. VVS1 and VVS2 (very very slightly included) feature inclusions invisible under 10x magnification. VS1 and VS2 (very slightly included) have inclusions visible under a loupe but not to the naked eye. SI1 and SI2 (slightly included) have inclusions visible under a loupe, and sometimes to the naked eye for SI2. I1, I2, and I3 (included) have inclusions visible to the naked eye, with an impact on brilliance.

Cut: cut quality is graded as Excellent, Very Good, Good, Fair, or Poor. An excellent cut maximises brilliance (reflected white light) and fire (dispersion into color). A poor cut renders a diamond dull, even when its color and clarity are high. Cut is the only one of the 4Cs controlled by human hands; the other three are entirely natural.

A gemological certificate (GIA, HRD, IGI) attests to the 4Cs. It does not prove that a diamond is natural, as laboratory-grown stones also carry their own certificates marked "laboratory-grown," but it confirms that the stated characteristics correspond to the stone. A seller who refuses to provide a certificate for a diamond of more than 0.5 carat should be treated with caution.

Cut glass (Bohemian crystal, Swarovski crystal) and rhinestone (lead glass with a metallic coating) have been imitating diamonds since the 18th century. Swarovski crystal contains 32% lead to increase its refractive index (1.7) and brilliance. Yet it remains far from a diamond (2.42).

Glass fails every test: persistent fogging (thermal conductivity 1 W/m·K), letters readable through the stone, low density (2.4 to 3.5 g/cm³ depending on lead content), hardness of 5.5 to 6 on the Mohs scale (scratched by a knife blade), round inclusions (air bubbles trapped during moulding).

Glass imitations often show a worn surface lustre. Glass is a soft material. After a few months of daily wear, the table (the top facet) loses its polish and becomes dull. The edges round off. A diamond retains its polish intact for decades.

Rhinestone (glass with an aluminium or silver coating on the pavilion) produces an intense but artificial sparkle. The metallic coating peels away over time, leaving dull patches. A real diamond never has a coating: its brilliance comes from its crystalline structure.

Jewellers refer to these imitations as "diamond simulants," "synthetic stones" (a misuse of the term, as glass is not a crystal), or "crystals." In France, the law prohibits the use of the word "diamond" alone to describe an imitation. One may say "diamond imitation," but not "synthetic diamond" when referring to glass.

A gemological certificate is a diamond's passport. Issued by an independent laboratory (GIA, HRD, IGI, SSEF, Gübelin), it confirms authenticity, origin (natural or synthetic), and the 4Cs. A full certificate costs between $120 and $360, depending on the size of the stone.

The GIA (Gemological Institute of America) is the global reference. A GIA certificate states: weight (carat), colour (D-Z scale), clarity (FL-I3 scale), cut (Excellent to Poor), exact dimensions (mm), proportions (table %, depth %), fluorescence (none to very strong), and origin (natural or laboratory-grown). The certificate includes a clarity diagram: a map of inclusions visible under 10x magnification.

Diamonds above 0.30 carat are often laser-inscribed on the girdle (the stone's outer edge) with the GIA certificate number, invisible to the naked eye but visible under a 10x loupe. You can verify this number on the GIA website to confirm that the certificate matches the stone.

A certificate does not prevent substitution: a dishonest seller may show you a GIA certificate for a 1-carat D VVS1 diamond, then set a 1-carat H SI2 diamond in the ring. Always compare the number inscribed on the diamond with the certificate number before purchasing. If the seller refuses this verification, walk away.

Lab-grown diamonds also come with their own certificates, bearing the notation "laboratory-grown" in large print. IGI and GCAL certify lab-grown stones. A certificate does not prove that a diamond is natural; it proves that the characteristics described correspond to the stone.

The sandpaper test (rubbing the stone against sandpaper) proves nothing. A diamond will scratch glass, but so will moissanite (Mohs hardness 9.25). This test needlessly damages the stone and does not distinguish between a diamond, moissanite, White Sapphire (Mohs 9), or topaz (Mohs 8).

The mutual scratching test (rubbing two diamonds against each other) is destructive. Even two genuine diamonds can scratch one another if their crystalline orientation is unfavourable. This test was used in the 19th century, when alternatives were rare. Today, with thermal testers and certificates, it serves no purpose.

The scanning electron microscope test (SEM) identifies the exact chemical composition. It is the definitive method for distinguishing natural, lab-grown, and imitation stones. However, an SEM costs $180,000 and is found only in specialised laboratories. For a retail purchase, GIA or HRD certificates are entirely sufficient.

The nitric acid test (immersing the stone in acid) only works on metals. Diamond, glass, cubic zirconia, and moissanite are all inert to nitric acid. This test distinguishes nothing and damages the setting.

Rely on the tests presented in this article: the fog test, the reading test, the loupe, weight, thermal tester, UV fluorescence, and double refraction. Together, they identify 99% of imitations without costly equipment or any risk to the stone.

We use exclusively certified natural diamonds in our creations. Every stone above 0.30 carat is accompanied by its gemological certificate. Our diamonds are set in 18K gold (yellow, white, or rose) or in 950 Platinum.

Our collection spans brilliant, princess, baguette, and pear cuts. We select diamonds in the G to H colour range (near-colourless), with VS2 to SI1 clarity (inclusions invisible to the naked eye), and Very Good to Excellent cut grades. This standard ensures exceptional brilliance at a considered price point: our 0.50-carat diamond rings start at $2,200.

For wedding rings, we offer grain settings (diamonds flush with the metal, invisible in profile) and claw settings (raised diamonds, maximising light return). The Moksha and Shanti designs feature baguette cut diamonds, a geometric form that captures light in clean, straight lines. The Indrani design features a central princess cut diamond, square with sharp corners, surrounded by round brilliant diamonds.

Not sure which size or shape to choose? Our team is available to answer your technical questions about diamonds by email or phone. We also offer virtual appointments so you can compare several stones on video before making your decision.

A gemological certificate is the diamond's passport. Issued by an independent laboratory (GIA, HRD, IGI, SSEF, Gübelin), it attests to authenticity, origin (natural or synthetic), and the 4Cs. A full certificate costs between $120 and $350, depending on the size of the stone.

The GIA (Gemological Institute of America) is the global benchmark. A GIA certificate states: weight (carat), colour (D-Z scale), clarity (FL-I3 scale), cut (Excellent to Poor), exact dimensions (mm), proportions (table %, depth %), fluorescence (none to very strong), and origin (natural or laboratory-grown). The certificate includes a clarity diagram: a map of inclusions visible under a 10x loupe.

Diamonds weighing more than 0.30 carat are often laser-inscribed on the girdle (the stone's outer edge) with the GIA certificate number. Invisible to the naked eye, it is visible under a 10x loupe. You can verify this number on the GIA website to confirm that the certificate corresponds to the stone.

A certificate does not prevent substitution: a dishonest seller may show you a GIA certificate for a 1-carat D VVS1 diamond, then set a 1-carat H SI2 diamond in the ring. Always compare the number inscribed on the diamond with the certificate number before purchasing. If the seller refuses this verification, walk away.

Synthetic diamonds also carry certificates, bearing the notation "laboratory-grown" in large type. IGI and GCAL certify synthetic stones. A certificate does not prove that a diamond is natural; it proves that the characteristics described correspond to the stone.

The sandpaper test (rubbing the stone on sandpaper) proves nothing. Diamond scratches glass, but so does moissanite (hardness 9.25 on the Mohs scale). This test unnecessarily risks damaging the stone and cannot distinguish between a diamond, moissanite, White Sapphire (9 Mohs), or topaz (8 Mohs).

The mutual scratching test (rubbing two diamonds against each other) is destructive. Even two genuine diamonds can scratch each other if the crystal orientation is unfavorable. This test was used in the 19th century, when alternatives were scarce. Today, with thermal testers and certificates, it no longer serves any purpose.

The scanning electron microscope test (SEM) identifies the exact chemical composition. It is the definitive method for distinguishing natural, synthetic, and imitation stones. However, an SEM costs $180,000 and is found only in specialized laboratories. For everyday purchases, GIA/HRD certificates are more than sufficient.

The nitric acid test (submerging the stone in acid) only works on metals. Diamond, glass, cubic zirconia, and moissanite are all inert to nitric acid. This test distinguishes nothing; it only damages the setting.

Stick to the tests presented in this article: the fog test, the reading test, the loupe, weight, thermal tester, UV fluorescence, and double refraction. Together, they identify 99% of imitations without costly equipment or any risk to the stone.

We use exclusively certified natural diamonds in our creations. Every stone above 0.30 carat comes with its gemological certificate. Our diamonds are set in 18K gold (yellow, white, or rose) or in 950 Platinum.

Our collection spans brilliant, princess, baguette, and pear cuts. We select diamonds graded G to H in color (near-colorless), VS2 to SI1 in clarity (inclusions invisible to the naked eye), and Very Good to Excellent in cut. This standard ensures optimal brilliance at a considered price: our 0.50-carat diamond rings start at $2,200.

For wedding rings, we offer grain settings (diamonds nestled flush within the metal, invisible in profile) and claw settings (raised diamonds that capture maximum light). The Moksha and Shanti designs feature baguette cut diamonds, a rare geometric form that captures light in clean, straight lines. The Indrani design features a central princess cut diamond, square with sharp corners, surrounded by round brilliant diamonds.

Not sure which size or shape to choose? Our team is available to answer your technical questions about diamonds by email or phone. We also offer virtual appointments to compare several stones by video before you make your decision.

Each test opens up a broader question. To master every facet of the stone, continue your reading with our dedicated guides.