Gemstone Thermal Properties


Stone feels cold to the touch because it has a high thermal inertia. Thermal inertia is of the thermal properties that gemologists can use for gemstone identification. “Coloured marble cold stone” by Pia Poulsen is licensed under CC By 2.0
Stone feels cold to the touch because of its high thermal inertia. Gemologists can use thermal inertia, as well as other thermal properties, for gemstone identification. “Coloured marble cold stone” by Pia Poulsen. Licensed under CC By 2.0.

Gemologists are severely restricted in analyzing and identifying gemstones because their testing methods must be non-destructive. This limits the measuring process 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 and must be 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 is 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 at room temperature, which includes gemstones.

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 is 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 with 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

Material

Thermal Conductivity (cal/cm ⁰C sec)

Specific Heat (cal/cm ⁰C)

Density (gm/cmᶟ)

Thermal Diffusivity (cm² / sec)

Thermal Inertia (cal/cm²  ⁰C sec½)

Gem Materials, Synthetics, and Simulants
Diamond

1.6-4.8

0.12

3.52a

3.79-11.4

0.822-1.42

Silicon carbide (synthetic)

0.215ᵇ

0.2*

3.17ᵃ

0.0339

0.369

Periclase (synthetic)

0.110ᵇ

0.2*

3.575ᵃ

0.154

0.281

Corundum: c axis

0.0834ᵇ

0.206

4.0ᵃ

0.101

0.262

                        a axis

0.0772

0.206

4.0ᵃ

0.0937

0.252

                        c axis

0.060ᶜ

0.206

4.0ᵃ

0.0728

0.222

Topaz:  a axis

0.0446

0.2*

3.53ᵃ

0.0632

0.177

               mean, Gunnison, Colorado

0.0269

0.2*

3.531

0.0381

0.138

Pyrite: Colorado

0.0459

0.136

4.915

0.0684

0.176

Kyanite: c axis

0.0413ᵇ

0.201

3.66ᵃ

0.0562

0.174

                  b axis

0.0396ᵇ

0.201

3.66ᵃ

0.0539

0.171

                  mean, Minas Gerais, Brazil

0.0338

0.201

3.102

0.0461

0.158

Hematite: Itabira, Brazil

0.0270

0.169

5.143

0.310

0.153

Spinel: locality unknown

0.0281

0.216

3.63ᵃ

0.0358

0.148

               Madagascar

0.0227

0.216

3.633

0.0288

0.133

Fluorite: locality unknown

0.0219

0.220

3.18ᵃ

0.0313

0.124

                  Rosiclare, Illinois

0.0227

0.220

3.186

0.0324

0.126

Sphalerite: Chihuahua, Mexico

0.0304

0.115

4.103

0.0646

0.120

Sillimanite: Williamstown, Australia

0.0217

0.203

3.162

0.0339

0.118

Andalusite: Minas Gerais, Brazil

0.0181

0.202

3.102

0.0289

0.107

Pyrophyllite: North Carolina

0.0194

0.2*

2.829

0.0343

0.105

Jadeite: Japan

0.0159

0.206

3.196

0.0242

0.102

              San Benito County, California

0.0110

0.206

3.350

0.016

0.0873

Gahnite: Colorado

0.0103

0.2*

4.163

0.100

0.102

Magnesite: Transvaal

0.0139

0.236

2.993

0.0198

0.0992

Rutile: c axis

0.0231ᵇ

0.189

4.2ᵃ

0.0291

0.135

               a axis

0.0132ᵇ

0.189

4.2ᵃ

0.0166

0.102

               mean, Virginia

0.0122

0.189

4.244

0.0153

0.0990

Grossular: Connecticut

0.0135

0.196

3.617

0.0188

0.0979

                     Chihuahua, Mexico

0.0134

0.196

3.548

0.0193

0.0967

                     Crestmore, California

0.0124

0.196

3.318

0.0190

0.0898

Quartz: c axis

0.0264ᵇ

0.196

2.65ᵃ

0.0578

0.125

                c axis

0.0264ᶜ

0.196

2.65ᵃ

0.0509

0.117

                a axis

0.0140ᵇ

0.196

2.65ᵃ

0.0270

0.0854

                a axis

0.0160ᶜ

0.196

2.65ᵃ

0.0308

0.0912

               mean, Jessieville, Arkansas

0.0184

0.196

2.647

0.0354

0.0978

Spodumene: Maine

0.0135

0.2*

3.155

0.0214

0.0923

Diopside: New York

0.0133

0.196

3.270

0.0208

0.0923

                     Madagascar

0.00969

0.196

3.394

0.0146

0.0802

Dolomite

0.0132

0.221

2.857

0.0209

0.0911

Olivine (peridot, Fo₈₆Fa₁₄)

0.0115

0.2*

3.469

0.0166

0.0893

Elbaite: Keystone, South Dakota

0.0126

0.2*

3.134

0.0202

0.0889

Talc, Quebec

0.0124

0.221

2.804

0.200

0.0878

Tremolite: Balmat, New York

0.0117

0.210

2.981

0.0186

0.0854

                      Ontario, Canada

0.0112

0.210

3.008

0.0177

0.0839

Amblygonite: South Dakota

0.0119

0.2*

3.025

0.0197

0.0850

Zircon: Australia

0.0109

0.140

4.633

0.0167

0.0839

Enstatite: (En₉₈Fs₂): California

0.0105

0.2*

3.209

0.0334

0.0821

Bronzite: (En₇₈Fs₂₂): Quebec

0.00994

0.2*

3.365

0.0148

0.0818

Material

Thermal Conductivity (cal/cm ⁰C sec)

Specific Heat (cal/cm ⁰C)

Density (gm/cmᶟ)

Thermal Diffusivity (cm² / sec)

Thermal Inertia (cal/cm²  ⁰C sec½)

Spessartine: Haddam, Connecticut

0.00811

0.2*

3.987

0.0102

0.0804

Datolite: Paterson, New Jersey

0.0106

0.2*

2.996

0.0177

0.0798

Anhydrite: Ontario, Canada

0.0114

0.187

2.978

0.0204

0.0796

Almandine: Gore Mountain, New York

0.00791

0.2*

3.932

0.0101

0.0789

Staurolite: Georgia

0.00828

0.2*

3.689

0.0112

0.0782

Augite: Ontario

0.00913

0.2*

3.275

0.014

0.0773

Pyrope: Navajo Reservation, Arizona

0.00759

0.2*

3.746

0.0101

0.0754

Andradite: Ontario, Canada

0.00738

0.2*

3.746

0.00984

0.0744

Smithsonite: Kelly, New Mexico

0.00612

0.2*

4.362

0.00701

0.0731

Beryl: c axis

.0131ᵇ

0.2*

2.70ᵃ

0.0243

0.0842

          a axis

.0104ᵇ

0.2*

2.70ᵃ

0.0193

0.0750

          mean, Minas Gerais, Brazil

0.00953

0.2*

2.701

0.0176

0.0718

Calcite: Chihuahua, Mexico

0.00858

0.218

2.721

0.0145

0.0713

Axinite: Baja California

0.00767

0.2*

3.306

0.0116

0.0712

Prehnite: Paterson, New Jersey

0.00854

0.2*

2.953

0.0145

0.0710

Rhodochrosite: Argentina

0.00731

0.184

3.584

0.0111

0.0695

Flint: Brownsville, Ohio

0.00886

0.2*

2.618

0.0169

0.0681

Epidote: Calumet, Colorado

0.00627

0.2*

3.413

0.00919

0.0654

Petalite: Zimbabwe

0.00856

0.2*

2.391

0.0179

0.0640

Clinozoisite: Baja California

0.00574

0.2*

3.360

0.00854

0.0621

Idocrase: Chihuahua, Mexico

0.00576

0.2*

3.342

0.00863

0.0620

Sphene: Ontario, Canada

0.00558

0.188

3.525

0.00845

0.0607

Iolite: Madagascar

0.00650

0.2*

2.592

0.0126

0.0580

Zoisite: Liksviken, Norway

0.00513

0.2*

3.267

0.00785

0.0579

Aragonite: Somerset, England

0.00535

0.209

2.827

0.00906

0.0562

Microcline: Amelia, Virginia

0.00621

0.194

2.556

0.0126

0.0554

                   Ontario, Canada

0.00590

0.194

2.558

0.0119

0.0541

Albite: (Ab₉₉An₁): Amelia, Virginia

0.00553

0.202

2.606

0.0105

0.0540

Serpentine (lizardite): Cornwall, England

0.00558

0.2*

2.601

0.0107

0.0539

Orthoclase: Goodspring, Nevada

0.00553

0.2*

2.583

0.0107

0.0534

Sodalite: Ontario, Canada

0.00600

0.2*0

2.326

0.0129

0.0528

Lepidolite: Dixon, New Mexico

0.00460

0.2*

2.844

0.00807

0.0512

Anorthite (Ab₄An₉₆): Japan

0.00401

0.196

2.769

0.00737

0.0467

Flour-apatite: Ontario, Canada

0.00328

0.195

3.215

0.00522

0.0454

Chlor-apatite: Snarum, Norway

0.00331

0.195

3.152

0.00539

0.0451

Labradorite (Ab₄₆An₅₄): Nain, Labrador

0.00365

0.2*

2.701

0.00676

0.0444

Barite: Georgia

0.00319

0.113

4.411

0.00639

0.0399

Apophyllite: Poona, India

0.00331

0.2*

2.364

0.00699

0.0396

Leucite: Rome, Italy

0.00274

0.2*

2.483

0.00551

0.0369

Vitreous silica (General Electric)

0.00325

0.201

2.205

0.0074

0.0379

Hyalite: Spruce Pine, North Carolina

0.00290

0.2*

2.080

0.0070

0.0347

Glass: obsidian

0.00330ᵇ

0.2*

2.4ᵃ

0.00688

0.0398

           ordinary flint (lead)

.0018ᵇ

0.117ᵃ

3.5ᶜ

0.00440

0.0272

           very heavy flint (lead)

00.12ᵇ

0.117

4.5ᵃ

0.00228

0.0251

Metals
Copper

0.927

0.092

8.89

1.13

0.871

Silver 100%

1.00

0.056

10.5

1.70

0.767

Silver 96%, gold 31% (weight)

0.237

0.048*

12.3

0.401

0.374

Silver 34%, gold 66% (weight)

0.152

0.040*

15.5

0.245

0.307

Gold 100%

0.707

0.031

19.3

1.18

0.650

Aluminum

0.485

0.214

2.7

0.839

0.529

Platinum

0.166

0.032

21.4

0.242

0.337

Platinum, 10% iridium

0.074

0.032*

21.6

0.107

0.226

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 another reference is indicated by a superscript letter, 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.