Are Diamond Testers Accurate? (How Do They Work Exactly) 

Diamonds can cost as much as a house – sometimes, even more. Likewise, diamonds are so durable that they’ll likely last even longer than your house and then millions of years after. Before the return policy expires, you may want to verify the legitimacy of a potentially lifelong commitment. 

Luckily, if you don’t trust your sellers’ word at purchase, diamond testers are generally accurate and trustworthy. Yet even with the best equipment, nothing is ever perfect, and user-errors can still abjure the truth. Diamond testers can spot many fausse, but moissanite in particular is not easy to identify.

Similar to a blood test, the probability of one test being inaccurate is greater than the probability of multiple tests being inaccurate.

However, the increased accuracy from multiple tests may not be enough to justify multiple tests. To understand the controversy and potential inaccuracies, I’m going to walk through the different types of tests and roots of error.

The Basics of Diamond Testers

A diamond tester does exactly as its name suggest. It tests the legitimacy of a potential diamond. A diamond tester is generally a small, portable device that checks the physical properties of a stone and compares it to the known physical properties of diamonds.

While traditional methods of testing diamonds are timeless, the actual devices themselves were invented in the 1990s. Though mostly accurate, they are only decades old and could still be improved in the future.

Anyone can, (and probably should), purchase a diamond tester. They are incredibly easy to use and relatively inexpensive. The cost of the average diamond tester is still less than 1% the expected cost of a diamond.

Diamond testers are especially useful for jewelers and professionals. If you don’t want to buy a tester yourself, you can go to your local jeweler and have them test it for you. They may even do it for free.

There are more advanced diamond testers that are not popular for consumer use. One example is a UV test to differentiate between natural and lab grown diamonds. This detector can cost between $7000 and $12000 dollars.

Though inexpensive and relatively easy, having a professional use their own diamond tester could reduce the potential for user-error.

Stones That a Diamond Tester Can Detect

Natural diamonds and lab diamonds are both pure forms of carbon; they have the same carbon composition and chemical structure. Natural diamonds and lab-grown diamonds are chemically identical; both can be tested with a diamond tester.

There are instances where a genuine lab diamond may show up as negative. Some lab diamonds contain less nitrogen than naturally occurring diamonds, which makes them less electroconductive. They, then, may falsely fail the test.

Diamond testers can also be used for other gemstones. They work especially well for testing emerald and sapphire, but the same test can be used on many of stones.

How Diamond Testers Work

There are two main physical properties that diamond testers test: thermal conductivity and electrical conductivity. Each type of standard diamond tester has a similar physical appearance — a small box with a protruding tip to test the stone.

Diamonds have extreme physical properties that make them stand out from other stones, and are therefore, easy to test.

Thermal Conductivity Diamond Testers

A thermal conductivity diamond tester tests the thermal conductivity of a stone — the rate that heat is transmitted through a stone. Diamonds have the highest thermal conductivity of any known material. Diamonds pass heat through them faster than any other material.

Thermal conductivity test

A thermal conductivity diamond tester works by transferring heat to the stone that is being tested. When turned on, the tip heats up; when placed against the stone, heat is transferred. The diamond tester then detects the rate that heat is transferred through the stone.

Since diamonds have the highest thermal conductivity, no other stone should reach the same level on the thermal conductivity scale. The main issue is moissanite, a stone made up of silicon carbide – a very similar chemical composition to diamond.

A thermal conductivity may not register the small difference in thermal conductivity between moissanite and diamond.

Electrical Conductivity Diamond Testers

Similar to a thermal conductivity tester, an electrical conductivity diamond tester tests the rate that electricity is transmitted through a stone. Diamonds are not electricity conductors; they are electricity insulators.

Electrical conductivity diamond test

An electrical conductivity diamond tester works by transferring electricity to the stone that is being tested. When turned on, electricity is transferred to the tip and then to the stone. The tester than detects the rate the stone conducts electricity.

Since diamonds have a very low thermal conductivity, (not the lowest known material but lower than other stones), no other stone should register the same level on the electrical conductivity scale.

The electrical conductivity between diamond and moissanite is notably different; an electrical diamond tester is more accurate in spotting the difference.

A notable exception to the accuracy of an electrical conductivity tester is blue diamond. Blue diamonds contain boron, which is an electric conductor. If testing a legitimate blue diamond, an electrical conductivity test will likely display a false negative.


Many of the latest market testers are multi-testers; they test both thermal and electrical conductivity.

This can add both extra validity and reduce user error. Multi-testers can spot moissanite and warn you if you touch metal rather than stone, which other testers don’t always have.

This increased quality currently comes at an increased price, which may not be worth it since electrical conductivity testers produce similar accuracy.

Moissanite is both chemically and optically similar to diamonds, making them a known culprit of deception. They have a very high thermal conductivity; they may pass a thermal conductivity diamond test. Moissanite is much more electroconductive than diamond and will not pass an electrical conductivity diamond tester.

A diamond tester that only tests thermal conductivity will not be able to falsify moissanite. While a general electroconductivity test can spot moissanite, there are specific moissanite testers. They could be worth the added validity.

Moissanite Testers

Moissanite testers test electrical conductivity, but with a higher degree of precision for this specific controversy.

Does Diamond Tester Price Correlate With Accuracy?

Aside from the price difference between a thermal and electric conductivity tester, increased accuracy is generally not enough to justify increased cost.

Lower-cost diamond testers are reliable as is; they are reliable enough to be used by professionals. The increased accuracy from a significantly more expensive test is so marginal that it isn’t worth it for general consumers.

When you go to purchase a diamond tester, many standard, lower-end testers are evasive about what they are testing. When in doubt, it’s safe to assume the diamond tester is testing thermal conductivity.

Thermal conductivity tests are accurate in almost every case except moissanite. Lower-cost thermal conductivity testers will likely not be able to differentiate moissanite from diamond.

Since moissanite and diamond are so optically similar, it may be worth the extra cost and effort to have an electrical conductivity or multi-test.

The issue with thermal conductivity testers is false positive for moissanite. On the contrary, false negatives aren’t typically an issue. If you have suspicion that a stone is fake and want a rapid answer, a thermal conductivity test could be the perfect solution.

Electrical conductivity tests are significantly more accurate and are still relatively inexpensive. The only notable potential for error is blue diamonds. Blue diamonds are often optically different from white diamonds; the chance of unknowingly having a blue diamond is quite slim.

Anyone can purchase a diamond tester. The least expensive are thermal testers, which go for as little as $15.

Diamond testers that explicitly test electrical conductivity generally go for around $200. A multi-test diamond tester, one that tests both thermal and electrical conductivity, also goes for around $200.

Since diamond is on extreme ends of the spectrum for relative thermal and electrical conductivity, virtually any one can test diamonds with accuracy.

Many jewelers and other professionals carry around electroconductivity testers – the same ones that are typically purchased for household use. Jewelers and professionals don’t typically use solely thermal conductivity testers because the presence of moissanite is so common.

Higher-end diamond testers, testers that go for thousands of dollars, conduct the same tests with more precision. Every conductivity tester at every end of the spectrum is programmed to assess conductivity and compare it to diamond. More advanced testers can have hundreds of reference marks to get a very precise and accurate conductivity threshold.

The extra precise test does not significantly increase accuracy in determining whether the diamond is real or fake. The extra threshold test may be used to spot impurities within a natural, authentic diamond. However, those impurities don’t mean the stone is a fake diamond.

A high-end diamond tester could be used by professionals for pricing precision and research, but the results are not of any significance to the general consumer.

Another way to test diamonds is via refractometer. A refractometer tests the ability and speed that a stone reflects light. Diamonds have a very high refractive index; they reflect light very quickly.

Refractometers are a tool that may be used by advanced professionals, but also don’t offer any additional accuracy or value to the general public.

The increased accuracy of an electrical conductivity tester is enough to justify an increased price, but the precision gained from higher-end conductivity tests is of little value to most.

How to Use a Diamond Tester

The most common at-home diamond tester is a thermal conductivity tester. Though the procedure for other testers is basically identical.

After extracting the diamond tester from the box and removing the plastic tip cap. You turn the device on by sliding the volume to the right. You then wait about thirty seconds; the device will signal when the lamp is ready.

Then you identify the stone you are going to test. There is a calibration chart on the back of the tester for the stone you’ve decided to test. Based on the size (in carats) and temperature of the stone, you use the chart to find the corresponding calibration number. 

When you choose a calibration number, you turn the volume button to the corresponding number of bars. There is a shiny panel on the back side of the tester. Your skin must be touching the panel.

Then pick up the stone you are testing and bring the tip of the device to the stone. If you touch metal instead of the stone, it will make an ongoing beeping sound. If the stone is a diamond, all of the bars will light up.

If you are looking to test another stone using a diamond tester, the same procedure applies. However, the necessary calibration and number of bars to verify legitimacy will vary.

A Few Things to Consider Before Buying a Diamond Tester

You can purchase a new diamond tester for about $15. In comparison to the cost of diamonds, the cost is a drop in a bucket. The device is also relatively small; it doesn’t take up precious interior space.

While diamond testers are both inexpensive and small, there is a level of inconvenience. Sometimes having another trinket sitting around the house collecting dust can be an annoyance.

Additionally, diamond testers are battery operated. To use a diamond tester, you may have to purchase a pack of batteries, which can be an unnecessary nuisance.  

Jewelers and other diamond professionals undoubtedly have diamond testers. You can have them test diamonds or other stones for you. Though they may charge a small fee, having them test it could alleviate the annoyance of buying another device.

On the contrary, if you have a lot of stones to test, it may be most convenient to have your own diamond tester. Diamond testers can test other stones and can be especially helpful for testing emerald and sapphire. If the hassle of going to the nearest jeweler outweighs the hassle of doing it yourself, maybe do it yourself.

Another issue to consider is the potential for user-error. Though diamond testers are fairly easy to use, there are factors that a nonprofessional may not know to look for.

One example is the heat of the stone. A cold diamond could alter the results of the test and yield a false negative or positive.

Another example is for other stones that may have a higher likelihood of false testing. Rubies are known for high error from diamond testers, something the average person likely wouldn’t know.

Understandably, lack of professional knowledge could yield error that justifies having a professional test it for you.