Gemstone identification can be an arduous problem, and getting it right is essential for the people in the trade. Recently, electronic gem testers have come in the market that promise to simplify this problem. These are small, handheld devices with a small probe that is placed against the gemstone.

These devices first entered the market as diamond testers, and helped solve the problem of distinguishing natural diamond from cubic zirconia (a synthetic diamond stimulant). An electronic diamond tester is, in fact, a portable device that we can put in our pocket. The tester has a small, needlelike tip that needs to be placed firmly on the stone under test. If the diamond is real, the device indicates that on its display, often with an aural alert.

How it works

Diamond is an extremely good conductor of heat, and this property has been the basis of one of the oldest tests for diamond—touch a diamond with your lip and it will feel cold! Diamond testers’ working principle is based on the fact that different gemstones conduct heat differently. Heat will pass through a diamond in a different way than it would through a cubic zirconia or glass. The diamond tester will detect the rate at which heat moves through the stone and tell whether it is a real diamond or not.

As diamond is an exceptional thermal conductor (and cubic zirconia is not), the first diamond testers used a heated probe to detect heat dissipation. These were very effective at distinguishing natural diamond from cubic zirconia. But then another diamond simulant called moissanite entered the market, and diamond testers were not able to detect it (moissanite is also a very good thermal conductor).

Moissanite is a naturally-occurring mineral that was first discovered more than 100 years ago. At first glance, it looks very similar to diamond, and these two stones also seem to have the same physical properties. Although many people think of it as a substitute for diamond, moissanite’s chemical composition is quite different—diamond is made up of carbon, whereas moissanite is a form of silicon carbide.

This recant called for improved testers that use electrical conductivity to test stones. Latest diamond testers on the market include both thermal and electrical conductivity test modes. The advantage here is that you only need one device instead of two, and it costs slightly less to buy the two-in-one tester/multi-tester (heat for diamond and electricity for moissanite) rather than two separate testers.

Type II diamond tester.

Moissanite testers work by measuring electrical conductivity through the stone (diamond is not electrically-conductive, and moissanite is). However, there is a very rare type of diamond (type II diamond) that has an unusual chemical composition that makes it electrically conductive. So it will show as moissanite on a two-in-one tester/multi-tester. Fortunately, type II diamond testers are now available but cost a few extra bucks.

Role of UV light

Some diamond testers (Fig. 3) have a built-in ultraviolet (UV) light source, and this has led to the logical assumption that UV light can be used for testing diamonds. Note that, there is absolutely no way we can distinguish a diamond from a non-diamond using UV light.

UV light does, however, have some use for professional gemologists because it can give an indication of probability when comparing a natural diamond with a synthetic one. And, UV light makes a difference when testing a moissanite using a moissanite tester or two-in-one tester/multi-tester. Moissanite testers that do not work on some odd moissanites work perfectly when the stone is exposed to UV light. This is because, according to the laws of Quantum Physics, UV light can make a noticeable difference to electrical conductivity.

Gem tester electronics

Fig. 4 shows the reference system diagram of a gem tester that uses high voltage to quickly test a gemstone for legitimacy. This basic concept is adaptable for determining a gem type based on its electrical conductivity.

In principle, an electronic circuit (including the gem under test) as part of a circuit path is used to measure its electrical conductivity and, hence, the gem type. The procedure of determining whether a gem is moissanite or synthetic diamond involves providing a high voltage across a gem surface, greater than a breakdown voltage, and measuring a minuscule current that flows through the gem.

Here, the first (probe) and second (clip) contacts of the circuit couple the high voltage to the gem under test, while a low-impedance detector circuit is used to flag when the contacts are erroneously contacting each other during measurement. The amount of current flowing through the device is measured using a high-impedance resistor referenced to ground. Voltage on the resistor is measured and compared to a predetermined threshold voltage. Measurements above the threshold voltage indicate that the gem is conductive, and that the gem therefore is a synthetic diamond or moissanite.