Destructive Mineral Tests: Hot Point Testing

Keep in mind that hot point testing is a destructive procedure. Mineralogists can use to identify organic material (we can explain this more fully below), but gemologists should conduct this test on finished gems only as a last resort for identification.

What Do You Need for a Hot Point Test?

You can purchase electrically heated hot points or you can make your own. Just insert the blunt end of a needle into a cork. To use, simply heat the needle over a small flame until the tip turns a dull red.

Heavy Liquid Warning

Do not use heavy liquidson a gem before hot point testing. In fact, it's not a good idea to use heavy liquids for specific gravity testing on any porous stone. They will sweat out with heat and are very toxic to breathe.

Hot Point Testing for Plastic

1. Heat the tool until the tip glows dull red.
2. Place the stone close to your face and touch the hot point to an inconspicuous spot. (On a faceted gem, a spot near the girdle will usually do).
3. Hold it there for a second or less and then smell the smoke.

Amber smells resinous, like incense.

Coral and shells have a protein smell, like burning hair.

Jet has an oily smell, like coal, tar, or asphalt.

Most plastics have an acrid smell, but very few have a protein odor. Stones that are coated with plastic will have much less odor than solid ones. Try this on something you know is solid plastic first. It just takes a little practice to make this distinction.

Hot Point Testing for Wax Treatments

A thin layer of wax improves the appearance of many stones, particularly those that are hard to polish. This includes jade, carvings with hard to reach areas, and soft stones like turquoise and lapis lazuli. The wax sometimes has a dye added to enhance the color of the gem as well.

A hot point will tell you if a stone has been coated with wax. The procedure is similar to testing for plastic. Start by heating the tool to a dull red. However, this time try not to touch the stone. Just hold it very close, about 1/16 of an inch or so from stone. Observe the surface of the stone. If it's coated, you'll see the wax liquefy and bead up like perspiration.

What Is The Difference Between Gemology and Mineralogy For Hot Point Testing?

While gemologists reserve hot point testing for identifying organic or synthetic materials (and even then, it's used sparingly!), mineralogists rarely use hot point testing due to its limited application to inorganic materials. Most inorganic minerals are heat-resistant and do not melt or burn at temperatures achieved by hot point tools.

Occasionally, mineralogists may use hot point testing minerals with organic components (like bitumen or coal) or for rough specimens suspected of containing heat-sensitive inclusions. However, inorganic minerals rarely produce reactions like odors, melting, or burning when exposed to heat, so the test has limited diagnostic value in mineralogy.

Editor's Note: The original article has been updated and adapted for the Mineralogy Certification course.

Donald Clark, CSM IMG

The late Donald Clark, CSM founded the International Gem Society in 1998. Donald started in the gem and jewelry industry in 1976. He received his formal gemology training from the Gemological Institute of America (GIA) and the American Society of Gemcutters (ASG). The letters “CSM” after his name stood for Certified Supreme Master Gemcutter, a designation of Wykoff’s ASG which has often been referred to as the doctorate of gem cutting. The American Society of Gemcutters only had 54 people reach this level. Along with dozens of articles for leading trade magazines, Donald authored the book “Modern Faceting, the Easy Way.”

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