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 on diamonds vs cubic zirconia and glass.

These testers cost between £35 and £130 depending on 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 and moissanite, as moissanite has an almost equally high thermal conductivity (120 W/m·K). Some more recent 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 over 0.5 carat outside a certified channel, investing around £70 in a combined tester is a sensible 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.

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 on diamonds versus cubic zirconia and glass.

These testers cost between £35 and £130 depending on 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 an almost equally high thermal conductivity (120 W/m·K). Some more recent 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 a certified channel, investing around £70 in a combined tester is a reasonable precaution.

The 4Cs (Carat, Colour, 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 (flawless, colourless) for £850 is not being truthful: such a diamond is worth between £68,000 and £100,000.

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

Colour: the GIA scale runs 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 colour" 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 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 sometimes to the naked eye in the case of 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. Excellent, Very Good, Good, Fair and Poor. An excellent cut maximises brilliance (reflected white light) and fire (dispersion into colour). A poor cut renders the diamond dull, even when its colour and clarity grades are high. Cut is the only one of the 4Cs determined by human skill; the other three are entirely natural.

A gemological certificate (GIA, HRD or IGI) attests to the 4Cs. It does not alone prove that a diamond is natural (synthetic stones also carry 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.

Approximately 30% of natural diamonds emit a blue glow under UV light (wavelength 365 nm). This fluorescence is caused by traces of nitrogen within the crystal lattice. It is not, on its own, an indicator of authenticity or imitation, but it does offer a useful clue.

HPHT synthetic diamonds (high pressure, high temperature) often display orange, yellow or green fluorescence under UV light, rarely blue. CVD synthetic diamonds (chemical vapour deposition) typically exhibit a characteristic orange-red fluorescence. A blue fluorescence is an indication, though not proof, of a natural diamond.

Cubic zirconia never fluoresces under 365 nm UV light. If you expose your stone to a UV banknote detection lamp and it remains inert, this is not a sign of imitation: 70% of natural diamonds do not fluoresce. However, if it emits a vivid orange glow, it is most likely an HPHT synthetic.

A note of nuance: strong or very strong fluorescence can affect the value of a natural diamond, giving the stone a slightly milky appearance under intense natural light. Diamonds graded D, E or F (colourless) with strong fluorescence typically sell for 10 to 15% less than non-fluorescent stones. Diamonds graded I, J or K (slightly tinted) with medium fluorescence may actually benefit slightly: the blue glow offsets the yellow tint, making the stone appear whiter.

Equipment: a 365 nm UV lamp (known as "long wave") costs between £15 and £25. UV lamps at 254 nm (short wave) produce different results and are less reliable for diagnostic purposes.

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 £85 and £250, depending on the size of the stone.

The GIA (Gemological Institute of America) is the global reference standard. 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 micro-laser-engraved on the girdle (the stone's outer edge): the GIA certificate number, invisible to the naked eye, is visible under 10x magnification. 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 engraved on the Diamond with the certificate number before purchase. If the seller refuses this verification, walk away.

Synthetic diamonds also carry their own certificates, bearing the mention "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.

Cubic zirconia (zirconium oxide, ZrO₂) is the most widespread imitation of the Diamond. Created in a laboratory setting since 1976, it costs one thousandth of the price of a Diamond: a 1-carat cubic zirconia is worth £5 to £18, while a natural 1-carat Diamond of average quality is worth £3,400 to £5,000.

Visually, cubic zirconia sparkles. However, it fails every physical test of the Diamond: low thermal conductivity (persistent fogging), low refractive index (letters readable through the stone), high density (60% heavier), absence of UV fluorescence, hardness of 8 to 8.5 on the Mohs scale (scratches over time).

To the naked eye, cubic zirconia can be identified by its brilliance, too white, too uniform. A Diamond has an adamantine, metallic brilliance with grey-white reflections. Cubic zirconia has a glassy, softer sparkle, lacking the depth of a Diamond. The facet edges of an older cubic zirconia (worn for more than five years) are slightly rounded and worn down. The edges of a Diamond remain crisp and sharp.

Cubic zirconia should not be confused with natural zircon (ZrSiO₄), a rare gemstone used in fine jewellery. Natural zircon (density 4.6 to 4.7 g/cm³, hardness 7.5 on the Mohs scale) bears little resemblance to a Diamond. It is synthetic cubic zirconia that imitates the Diamond.

If you purchase a piece of jewellery for under £85, it most likely contains cubic zirconia. This is not a deception if the seller is transparent. It becomes a deception when the seller presents them as natural Diamonds.

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 18ct 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 outstanding brilliance at a considered price: our 0.50-carat Diamond rings start from £1,550.

For wedding rings, we offer grain settings (diamonds flush with the metal, invisible in profile) and claw settings (raised diamonds, allowing maximum light). 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.

Unsure which size or shape to choose? Our team is available to answer your technical questions about diamonds by email or telephone. We also offer virtual appointments, so you can compare several stones by video before making your decision.

A gemological certificate is the Diamond's passport. Issued by an independent laboratory (GIA, HRD, IGI, SSEF, or Gübelin), it attests to authenticity, origin (natural or synthetic), and the 4Cs. A full certificate costs between £85 and £250, 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 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 10x magnification. 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 engraved on the Diamond with the number on the certificate before purchasing. If the seller refuses this verification, walk away.

Synthetic Diamonds also come with their own certificates, bearing the designation "laboratory-grown" in large print. IGI and GCAL certify synthetic stones. A certificate does not prove that a Diamond is natural; it proves that the characteristics described match the stone.

The sandpaper test (rubbing the stone against sandpaper) proves nothing. A Diamond scratches glass, but so does moissanite (hardness 9.25 on the Mohs scale). This test damages the stone unnecessarily 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 unfavourable. This test was used in the nineteenth century, when alternatives were scarce. Today, with thermal testers and certificates available, it serves no purpose.

The scanning electron microscope (SEM) test identifies the exact chemical composition. It is the definitive method for distinguishing natural, synthetic, and imitation stones. However, an SEM costs around £130,000 and is found only in specialist laboratories. For everyday purchases, GIA or HRD certificates are entirely sufficient.

The nitric acid test (immersing the stone in acid) works only 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 18ct gold (yellow, white, rose) or in 950 Platinum.

Our collection spans brilliant cut, princess cut, baguette cut, and pear shapes. We select Diamonds in the G to H colour range (near-colourless), with a clarity of VS2 to SI1 (inclusions invisible to the naked eye), and a cut grade of Very Good to Excellent. This standard ensures outstanding brilliance at a considered price point: our 0.50-carat Diamond rings start from £1,550.

For wedding rings, we offer grain settings (diamonds embedded within the metal, invisible from the side) and claw settings (raised diamonds, maximising light). The Moksha and Shanti designs feature baguette cut diamonds, a rare geometric shape that captures light in straight lines. The Indrani design carries a central princess cut diamond, square with sharp corners, surrounded by round brilliant cut diamonds.

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

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