Gemstone Thermal Properties

The Limitations of Gemological Testing Methods

Gemologists are severely restricted in analyzing and identifying gemstones because their testing methods must be non-destructive. This limits measurements to the areas of optics (including spectroscopy, luminescence, and so forth), specific gravity, and inclusions. Gemologists don't routinely measure hardness on cut gems, since, again, that damages the gemstone.

Moreover, the instrumentation used in this field must be simple enough to be learned by people with no real scientific training as well as affordable. Much of the gemological literature these days reports measurements on gems made with various kinds of advanced instrumentation, such as ultraviolet absorption spectroscopy, X-ray fluorescence analysis, and even electron paramagnetic resonance. This is well and good for the literature but of little practical value for the working gemologist and/or appraiser.

For these reasons, it's important to explore the potential of any possible diagnostic method of gemstone analysis that is inexpensive, simple, and non-technical. One such method is the measurement of thermal properties, such as specific heat, thermal diffusivity, thermal conductivity, and thermal inertia.

Heat Energy Transfer Methods

Heat energy may be transferred in one of three ways: radiation, convection, and conduction. Sunlight is an example of radiation, while the creation of currents in a pot of boiling water is an example of convection. The third method of heat transfer, conduction, is the most relevant to solid materials, including gemstones, at room temperature.

Gemstone Identification and Thermal Properties

There are four thermal properties of potential interest for gemstone identification, three of which are mathematically interrelated. The best one for gem testing, thermal inertia, is the one that can most easily be measured with simple instrumentation. All four of these properties are defined below.

Specific Heat

Specific heat is the amount of heat required to raise one gram of a substance one degree Celsius. This is a constant for a given substance but varies from substance to substance. However, it varies little from one gemstone material to the next. Therefore, it's not especially useful for identification purposes.

Thermal Diffusivity

Thermal diffusivity is a measure of the velocity of heat flow in a material. If heat is applied to a substance, some of the heat energy goes into raising the temperature of the substance. The degree of heat energy that goes into raising the temperature depends on the specific heat of the material. The rest of the heat energy diffuses away from the point where the heat is being applied. The higher the thermal diffusivity of a material, the faster it will pass heat energy from one point to another.

Thermal Conductivity

Thermal conductivity, on the other hand, is a ratio of the flow of heat through a given thickness of material to the temperature difference across this thickness. It turns out that thermal conductivity is directional, just like refractive index, in all but isotropic (isometric or amorphous) materials. The symmetry of optical and thermal properties is usually the same. However, very few measurements on the variation of conductivity with direction have been made on gem materials.

Thermal Inertia

Thermal inertia is a measure of how quickly the surface temperature of a material can be changed by a flow of heat into the material. The higher the thermal inertia, the slower the surface temperature will rise when heat is applied. This is why materials, such as plastics, with a low thermal inertia feel warm to the touch. Body heat rapidly raises the surface temperature of such materials. Stone objects, on the other hand, feel cold to the touch because they have a high thermal inertia.

Using Diamond Probes

Thermal inertia is a directional property but lends itself to simple instrumentation for measuring a mean value. The various diamond probes on the market, including those made by GIA, Rayner, Kashan, and Ceres Corp., take advantage of this fact. Such probes consist of a temperature-difference sensor called a thermocouple and an adjacent thermal source, or resistance heater, surrounded by an insulated probe housing.

When using such instruments, take care to prevent drafts from affecting the readings. The probe tip is placed against the material being measured, in this case the gemstone facet, and a meter reading is obtained in about one second. This reading can be related to thermal inertia.

The commercial probes were developed specifically to distinguish diamond, which has a very high thermal inertia, from its imitations, such as cubic zirconia, with much lower thermal inertia.

You may encounter difficulties using commercial probes on very small stones. However, you can calibrate the instrument against small gems to avoid this problem.

Dr. Donald Hoover of the U.S. Geological Survey compiled the following table. Generally, it arranges the materials in order of decreasing thermal inertia. If accurate, quantitative probes become widely used, thermal inertia could become a very useful, easily measured parameter for gemstone analysis.

Thermal Properties of Gem Materials, Synthetics, and Simulants, as well as Some Metals at Room Temperature

Sources

From D. B. Hoover, "The GEM DiamondMaster and the Thermal Properties of Gems," Gems & Gemology, Summer 1983: 77-86. © Gemological Institute of America. Reprinted with Permission.

Unless a superscript letter indicates another reference, the values for conductivity and density were taken from K. Horai, "Thermal conductivity of rock forming minerals," Journal of Geophysical Research, 76 (5), 1971.

Values for specific heat, from R. A. Robie and D. R. Waldbaum, "Thermodynamic Properties of Minerals and Related Substances at 298.15 Degrees K and One Atmosphere Pressure and at High Temperatures," U.S. Geological Survey Bulletin, No. 1259, 1968.

Notes

* Assumed value; not found in the literature.

ᵃ R. Webster, 1982, Gems, 3rd ed. Hamden, Conn.: Butterworth & Archon.

ᵇ Chemical Rubber Company, 1966, Handbook of Chemistry and Physics. 47th ed. Boca Raton, Fla.: Chemical Rubber Company

ᶜ S.P. Clark, 1966, Handbook of Physical Constants, Memoir 97. Boulder, Colo.: Geological Society of America.

Factors Affecting Thermal Inertia Measurement

Quantitative measurement of thermal inertia may be difficult using instruments designed specifically to separate diamond from other stones. New devices specifically designed for such measurements will represent the next generation of thermal meters. When using devices to measure thermal inertia, keep in mind that surface quality, specifically the degree of flatness and polish, affects readings, as does degree of crystallinity and chemical composition, especially in solid solution series.