Thermal Conductivity

With the rapid advancements in the field of crystal growth in the past several years, materials produced for the purpose of simulating diamond have become more and more difficult to detect. Because the R.I’s (refractive index) of diamond and most simulants are over the limit of the conventional refracatometer, identification is more difficult.

In 1978, the Ceres Corporation, which is a large manufacturer of synthetic cubic zirconia, came out with completely new type of instrument for the separation of diamond from the various simulants. The instrument is termed a ‘thermal conductivity probe’ and operates on the principle of thermal (heat) conductivity. Diamond is unique in this property and even the most impure Type I stones still conduct heat many times quicker than the next best material.

Since the Ceres probe was developed many other companies have also developed similar instruments. They usually consist of a small box with a meter or lights to indicate whether or not the stone is diamond. A pen-like probe is attached to the box by a wire. The conductivity of the stone is tested by pressing the copper tip of the probe against the stone. An electrical current is passed through a thermister in contact with the stone which heats it up slightly. Then the current is cut off and sensors measure the speed with which it cools. Diamond, having the highest thermal conductivity, cools tip faster than any other material.

The advantages of the thermal conductivity probe are many. Any size stone from 0.03 carat up can be tested, mounted or unmounted. The quality of the polish or thin surface coatings does not affect the accuracy.

Advantages
- Stones down to 0.03 carat can be tested.
- Mounted or unmounted stones can be tested.
- Faceted or rough stones can be tested.
- Doublets can be tested if several parts of the stone are checked.
- Thin surface coatings on the stone do not affect the accuracy.
- Each stone takes only about 3 seconds to test.
- The test is extremely accurate, especially for materials which closely resemble diamond in appearance.

Disadvantages
- The probe only indicates whether or not a stone is diamond. It does not distinguish between various simulants.
- As with any instrument, there is chance for error if it is not used properly or if it was manufactured poorly.

The thermal conductivity probes offer the gemologist an accurate and quick means of distinguishing between diamond and its simulants. If used properly by a trained gemologist, the results are both accurate and repeatable.