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How do diamond testers work?

Checking a diamond with a diamond tester

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Are you intrigued by what diamond testers are and how they work? Read on to learn about the three main types of diamond testers, how they work and their limitations.

What is a diamond tester?

A diamond tester is a device used to determine whether a gemstone is a real diamond rather than a diamond simulant, such as cubic zirconia or moissanite. 

There are three main types of diamond testers available on the market:

Thermal Conductivity Testers

A thermal conductivity diamond tester
A thermal conductivity diamond tester

These testers measure the thermal conductivity of a diamond, which is the rate at which heat passes through the stone. Diamonds are exceptional conductors of heat, while other materials, such as cubic zirconia, moissanite, and glass, are poor conductors of heat. Thermal conductivity testers work by heating a small area of the stone and measuring how quickly the heat spreads through the stone. If the stone is a diamond, the heat will spread quickly, and the tester will indicate that the stone is a diamond.

Electrical Conductivity Testers

An electrical and thermal conductivity diamond tester
An electrical and thermal conductivity diamond tester

These testers use electrical conductivity to differentiate between diamonds and other materials. Diamonds are electrical insulators, meaning that they do not conduct electricity. On the other hand, cubic zirconia and moissanite are electrical conductors, meaning that they do conduct electricity. Electronic diamond testers work by sending a small electrical current through the stone and measuring the level of electrical conductivity. If the stone is a diamond, the current will not be conducted, and the tester will indicate that the stone is a diamond.

Acoustic Testers

These testers use sound waves to determine the authenticity of a diamond. Diamonds have unique acoustic properties not found in diamond simulants, such as cubic zirconia, moissanite, and glass. Acoustic testers work by sending sound waves through the stone and measuring how the sound waves travel through the stone. If the stone is a diamond, the sound waves will travel through the stone in a unique manner, and the tester will indicate that the stone is a diamond.

Are diamond testers reliable?

Diamond testers can provide a quick and convenient indication of whether a gemstone is likely to be a real diamond or a diamond simulant. However, their reliability depends on several factors:

Testing Method

Some simulants, such as moissanite, have thermal conductivity similar to diamonds, which can result in false positive readings on a thermal diamond tester.

Instrument Quality

The reliability of a diamond tester also depends on the quality and accuracy of the instrument itself. Higher-quality testers with better calibration and sensitivity are generally more reliable in providing accurate results.

Testing Conditions

Proper testing conditions are crucial for accurate results. The gemstone's surface must be clean and free from oils or residues that could affect the thermal conductivity reading. Additionally, the tester should be used according to the manufacturer's instructions and guidelines for optimal performance.

Would a lab grown diamond pass a diamond tester?

Lab grown diamonds have the same chemical composition and crystal structure as mined diamonds, consisting of carbon atoms arranged in a crystal lattice. As a result, they possess the same physical properties as mined diamonds and therefore pass a diamond test. This means that lab-grown diamonds are generally indistinguishable from natural diamonds using a diamond tester alone.

The limitations of diamond testers

Diamond testers are designed to differentiate between diamonds and diamond simulants, such as cubic zirconia or moissanite, but they are not foolproof and cannot provide a definitive determination of a gemstone's authenticity. 

Furthermore, while a diamond tester can indicate whether or not a gemstone is likely to be a diamond, it cannot distinguish between a lab grown vs natural diamond or provide information about the origin or specific characteristics or quality of a diamond, such as colour, clarity, or carat weight.

How to be sure you’re buying a real diamond

When purchasing a diamond, there are several steps you can take to ensure authenticity:

Buy from reputable sources

Purchase diamonds from reputable and trusted jewellers, whether a physical store or an established online retailer. Look for well-known and respected brands or jewellers with a solid reputation for selling genuine, ethically sourced diamonds.

Request diamond certification

Ask for a diamond grading report or certificate from a reputable gemological laboratory, such as the Gemological Institute of America (GIA) or the American Gem Society (AGS). These independent laboratories assess, and grade diamonds based on their 4Cs (cut, colour, clarity, and carat weight) and provide an objective evaluation of the diamond's characteristics.

Verify the diamond's characteristics

Review the diamond's grading report to ensure that the information matches the stone you are purchasing. Verify the 4Cs, as well as other details like the measurements, fluorescence (if any), and any identifying features.

Inspect the diamond with a loupe

Use a jeweller's loupe (a small magnifying tool) to examine the diamond closely. Look for any inclusions, blemishes, or unique identifying characteristics mentioned in the grading report. Real diamonds typically have slight imperfections.

Ask questions

Don't hesitate to ask the jeweller or retailer questions about the diamond's origin, sourcing practices, and any additional information they can provide about the diamond. A reputable seller should be transparent and willing to address your concerns.

Consider professional appraisal

If you are purchasing a high-value diamond, consider getting an independent appraisal from a certified gemologist or a qualified appraiser. They can provide an expert evaluation of the diamond's quality, value, and authenticity.

To learn more, read How To Tell If A Diamond Is Fake.


Diamond testers are devices used to determine the authenticity of a diamond. They work by measuring a diamond's electrical conductivity, thermal conductivity, or acoustic properties to distinguish it from other materials such as cubic zirconia, moissanite, and glass. 

While being handy tools to quickly identify diamond simulants, it is important to note that no diamond tester is 100% accurate, so unless you’re buying a diamond from a reputable jeweller, it is always best to have a professional gemologist perform a thorough examination of the diamond to determine its authenticity.

David Rhode
Together with Tim Ingle, David created Ingle & Rhode to offer a better alternative to the traditional luxury brands. Since 2007, we’ve provided our customers with genuinely ethical engagement rings, wedding rings and fine jewellery – free from conflict diamonds, dirty gold and child labour. With more than 16 years experience in the jewellery industry, David has deep expertise in diamonds, gemstones and jewellery design and manufacturing.