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How do you test a diamond?

There are several key ways that jewelers, gemologists, and consumers test diamonds to determine their quality and authenticity. The most common methods involve assessing a diamond’s visual appearance, using specialized equipment to analyze its optical and physical properties, and issuing certification reports from independent gemological laboratories. Testing helps ensure that a diamond is real and not an imitation, and that its characteristics align with its represented quality grade. This allows buyers to make informed purchasing decisions and have confidence in diamond sellers’ claims.

Visual Appearance

The simplest way to begin analyzing a diamond is by carefully observing it with the naked eye under good lighting conditions. An experienced jeweler or gemologist can gauge a lot about a diamond’s quality just from its visual attributes. Here are some things they look for:


Diamonds come in a range of colors, from colorless to light yellow, brown, or gray hues. The closer to completely colorless, the higher quality it is considered. Color is rated on an alphabetical scale from D (colorless) to Z (light yellow). Jewelers inspect body color by tilting a diamond under light. They look for differences in shading or cloudiness within the stone.


Most diamonds contain tiny natural imperfections called inclusions. The fewer inclusions, and the harder they are to see under 10x magnification, the higher the clarity grade. Jewelers use a loupe to scan for inclusions and blemishes, assessing their number, size, type, position, and visibility.


Cut refers to a diamond’s proportions, symmetry, and polish. Well-cut diamonds reflect light internally for maximum brilliance, fire, and scintillation. Jewelers check the precision of the faceting, as well as the angles, alignment, and symmetry of a diamond’s cut. They look for issues like large culets, table or girdle roughness, misshapen facets, or poor symmetry.

Carat Weight

A diamond’s carat weight helps determine its size and value. Jewelers weigh a diamond very precisely, usually to the hundredth of a carat. Weighing allows comparing the stated weight to the actual measured weight to check for accuracy.

Advanced Testing Equipment

While visual inspection can reveal a lot, professional gemologists use specialized tools and equipment to test diamonds more scientifically. Here are some of the key techniques and tools used:


Powerful gemological microscopes with magnifications from 10x to 60x help inspect a diamond’s characteristics in greater detail. Higher magnification aids detecting tiny flaws. Microscopes with special lighting modes like darkfield or fiber-optic lighting allow enhanced analysis of clarity features.

Ultraviolet Light Testing

Jewelers use UV lamps emitting longwave (365nm) and shortwave (254nm) UV to detect treatments, fluorescence, and whether a diamond is natural or lab-created. Natural diamonds usually fluoresce blue under longwave UV and inert under shortwave. Synthetics fluoresce differently. UV can reveal joined/fractured pieces, clarify cloudiness, and more.


This specialized instrument measures a diamond’s refractive index, helping identify its species. Real diamonds have a very high refractive index of 2.417. Refractometers easily distinguish diamond from simulants like cubic zirconia which have lower refractive indexes. Refractive index helps confirm authenticity.


A polariscope uses polarized filters to test a diamond’s optical properties like birefringence and strain. Diamonds display characteristic optical anomalies viewable through the polariscope. It can detect treatments like high-pressure high-temperature (HPHT) processing or clarity enhancements.


Spectrometers analyze the spectral properties of light interacting with a diamond to gather extensive data about its characteristics. Advanced spectrometers like those using Fourier transform infrared spectroscopy (FTIR) can identify specific inclusions and treatments.

Thermal Conductivity Tester

These testers measure a diamond’s thermal conductivity. Diamonds conduct heat extremely well compared to other gems. Testers can easily separate diamonds from simulants like moissanite based on thermal properties. Conductivity confirms genuineness.

Electrical Conductivity Meter

Diamonds are excellent electrical insulators, while some imposters are conductors. Electrical testing probes can distinguish conductivity differences, identifying fakes like cubic zirconia which conduct electricity. Lack of conductivity in a gem confirms diamond.

Certification Reports

Independent diamond grading laboratories issue diamond quality reports after thoroughly analyzing stones using sophisticated testing methods. Reports detail a diamond’s characteristics like weight, measurements, cut, color, clarity, polish, symmetry, and more. They represent an impartial, expert opinion of the diamond’s properties. Well-known labs include:


The Gemological Institute of America is the top lab worldwide. GIA introduced the 4Cs (cut, clarity, color, and carat weight) and diamond grading standards used today. Its report is most coveted and trusted in the industry.


The American Gem Society has strict diamond grading standards. Its cut grade is considered one of the most consistent and accurate. AGS pioneered the “light performance” cut grade based on computer modeling.


The European Gemological Laboratory has multiple lab locations. It was the first to grade cut quality for round brilliant diamonds on its reports. EGL grades are considered slightly more lenient than others.


The International Gemological Institute pioneered laser inscription of diamonds. Its reports focus heavily on clarity characteristics mapped via diagramming. IGI has labs worldwide and offers timely service.

Reports from major labs are recognized globally and provide assurance to buyers on a diamond’s quality specifications. They are the gold standard for reliable, independent assessment.


Professional diamond testing combines visual examination, scientific measurement with advanced instruments, and analysis by expert gemologists to determine a diamond’s authenticity, characteristics, treatments, and quality factors. Careful testing validates that a diamond matches the 4Cs grades and value represented by sellers. For major purchases, buyers should insist on an accredited lab report to verify any claims and feel confident investing in a diamond. With proper diligence, testing reveals the truth about a diamond and helps smart shoppers make informed decisions. For gemologists and jewelers, proper testing equips them to accurately represent diamonds to their customers. Ongoing advances in technology continue to improve testing and grading capabilities.

Method Type of Testing What It Tests
Visual Inspection Appearance Color, clarity, cut, carat, fluorescence
Microscope Magnification Clarity, inclusions, finish
UV Light Light Wave Exposure Treatments, fluorescence
Refractometer Optical Refractive index, identification
Spectrometer Spectral Analysis Inclusions, treatments, identification
Thermal Probe Thermal Properties Identification, conductivity
Electrical Probe Electrical Properties Identification, conductivity
Lab Report Expert Analysis Cut, carat, color, clarity, polish, symmetry, fluorescence, measurements

Diamond testing utilizes various methods from simple visual inspection to high-tech laboratory analysis. Each provides valuable insights into a diamond’s authenticity, characteristics, treatments, and quality. A combination of testing techniques helps verify properties and ensure truth in representing diamonds to buyers. Advanced testing remains an important capability allowing gemologists to fully evaluate diamonds. For consumers, requesting thorough testing and reports provides assurance that a diamond is accurately represented and graded. Smart shopping depends on careful inspection, scientific measurement, and unbiased expert analysis.