Jewellers' Network

Dangerous Instruments

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In this modern age we are used to having a machine, an instrument, a device or an app for just about any task imaginable.


So it is not surprising then that we find our trade turning to ‘machines’ for help when faced with the task of testing stones for their identity.

Last week alone we had five clients claiming their stones to be diamonds based on the famous diamond tester, and one lady in a panic because a sales assistant in a jewellery store had told her that the stones in her ring were not diamonds.

I am not suggesting that we refrain from using ‘machines’, but rather to encourage that we don’t forget to use observation and logic in the process. The two instruments that, in my opinion, are most likely to lead people astray are the diamond/moissanite testers and reflectometers, that are misleadingly marketed as digital refractometers.

Diamond/Moissanite testers

Firstly it is important to know what you are testing and what the limitations are. The first diamond testers worked by measuring thermal conductivity ie. how well the material dispersed heat.

This method works well but you have to adjust the sensitivity based on the size of the stone as larger stones will always disperse heat better than smaller ones. When Moissanite arrived on the scene we needed a new way to differentiate diamond as moissanite was able to pass the thermal conductivity test.

It was discovered that moissanite had a degree of electrical conductivity that was enough to differentiate it from non conductive diamond. There are some important limitations to be considered.

Firstly some diamonds conduct electricity like blues and some black stones that will fail when using a diamond tester.

Secondly the smaller the stone the less likely it is to test as diamond. And lastly when testing stones in a mount be aware that if the probe touches metal it will not test as diamond.

So now that we know what we are testing and its limitations I would encourage that we use these testers in conjunction with close observation and other traditional tests before making a final decision on a stone’s true identity.

Digital Refractometer/ Gem Tester     

Again it is important to understand what we are testing and its limitations. So here we are looking to learn the refractive index of a material which is basically its ability to bend light. Light bends when its speed changes, the more optically dense a material the slower light can pass through it and the higher the refractive index. This property also has an effect on the critical angle which is the angle where light changes from being transmitted into the gem to being reflected off the surface.

The most accurate and reliable way to measure a gem’s R.I. is using a refractometer which uses a high density liquid to create a contact between the dense glass hemisphere and the gem which allows one to accurately measure the critical angle and therefore the R.I. There is another instrument that can give one an idea of a material’s R.I. but it does so in a different way, it uses infrared light and measures the angle of reflection hence the name reflectometer.

Today they are marketed as digital refractometers or gem testers. There are some limitations and some advantages to this method.

Firstly the limitations, without the contact liquid the readings are less accurate and the condition of the polished surface has an effect on the result. When the surface has a good polish stones often get a much higher reading whilst stones with a poor polish get a low reading.

The advantage to this method is that you can get a rough idea of R.I.’s that are too high to get an accurate reading with the refractometer because it is limited by the contact liquid to R.I.’s below 1.9. That is why reflectometers were traditionally only used for stones whose R.I was higher than 1.9.

In conclusion I hope that we all remember to use the power of observation in conjunction with these instruments and not trust blindly in their results.