Thermal Testing of Gemstones

Posted on September 1, 2021 by Cara Williams, FGA

Most of us are familiar with the hand-held, thermal, diamond testing instruments that became commonplace starting in the early 1980s—an era when CZ was the pinnacle of diamond simulation and well-trained and equipped gemologists were comparatively rare. While the knowledge to separate diamond from its simulants has increased, so have the number of simulants, as well as the number of available instruments to identify them. The use of silicon carbide (synthetic moissanite) led to the development of electrical conductivity testers of similar appearance, but which worked on different physical properties—electrical rather than thermal. While today’s focus is firmly on identifying synthetic and treated diamonds, and Raman spectrometers are increasingly used to definitively identify all gem materials and their imitators, thermal testing instruments remain in use and are still sold due to their ease of use, affordability, and portability. The theory behind them, based on the laws of physics, is not well understood by gemologists. Thermal properties of gems are rarely mentioned in gemology texts, other than the highly subjective and unprofessional touch method, whereby glass feels cooler than plastic, and quartz feels colder than glass, however, these properties have been recognized since the 16th century. Even so, the various terminology and means by which heat is transferred through and across gems is not well understood. Some excellent papers addressing this topic were published in 1982 and 1983 by Dr. D.B. Hoover1,2, but they have largely been ignored, possibly due to being highly technical. Thermal test- ing instruments are commonly thought to test thermal conductivity or thermal resistance, but this is not the case. Thermal resistance, thermal inertia, and thermal conductivity are related, yet different properties. At this point, incorrect terminology is well-ingrained in common usage, but at a scientific level it is helpful to know that we are looking at a property best labeled as thermal inertia. Thermal inertia measures the rate at which the surface temperature of a material alters when a specific amount of heat per second is applied. This is simply the ability of a material to diffuse heat, and it is directly related to the strength, closeness, and type of chemical bond. Thermal inertia testers measure the rate at which a material can lower the temperature of the probe by diffusing the heat away from the heated metal tip. There are several additional factors that can affect readings, which will be addressed later in this article. Gem materials with a high thermal inertia like diamond, have strong, close, ionic, molecular bonds that serve to diffuse the heat away quickly. These close bonds also result in greater hardness. Ionic bonds are stronger than covalent bonds and other types of molecular bonding and result in harder gems. FIGURE 4. Green glass with induced inclusions imitating emerald, set in platinum with diamonds. Photo by Bear Williams.FIGURE 5. Green glass with native cuts imitating tourmaline. Photo by Bear Williams. For gemological purposes, it is not so important to understand the physics as to understand that this relation between hardness (as defined in gemology using the Mohs’ scale) and thermal inertia is proportional for the harder minerals. If we look at hardness in terms of the Mohs scale, this is valid from diamond down to CZ; but in terms of general hardness, it de- pends on crystal axis, type of bonds, and the elements that compose the mineral. This explains why Y/N testers will occasionally give false positives for hard materials such as corundum (9), which is next on the Mohs’ scale after diamond, and why an entirely new method of testing was required with the advent of synthetic moissanite (9.5) that could imitate diamond. CZ at 8.5 on the Mohs’ scale, is what most of the simpler thermal inertia testers are calibrated for, hence anything with a hardness over 8.5 is liable to yield false results, although this is easily checked by an astute, trained gemologist. Once the hardness is below that of CZ, the correlation between hardness and thermal inertia is less consistent, as molecular density and conductive elements in a ....

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