Step 3: Practical Gemology

Lesson 21

Hot point testing can determine if a gem material is natural or a plastic imitation. It can also reveal if a stone is treated with wax or plastic. 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 liquids on 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…