Destructive Mineral Tests: Acid Testing

Organic Gem Materials

Natural coral, shell, and pearl are carbonates. Since they release a protein smell from hot point testing, you can't use that test to distinguish them from plastic imitations. However, you can identify them by observing their reaction to acid.

Warnings

This is a potentially dangerous procedure for both stone and tester. Exercise great care. Keep in mind that acid testing is a destructive test that should only be conducted as a last resort.

Always Add Acid To Water

AAA - Always Add Acid to water! Never the other way around. If you pour water in acid, it will boil and splash out of the container. The acid will burn metal, wood, skin, etc. Memorize the AAA rule.

Pearl Warning

Since there are nondestructive ways to separate natural pearls from their imitations, acid testing isn't recommended for pearls.

Necklace Alert

When acid testing a strand of beads, make sure you don't get any fluid on the cord. The acids used for identification will damage cords.

Do Not Use Full-Strength Hydrochloric Acid

Full-strength hydrochloric or muriatic acid isn't recommended for gem and mineral testing. Diluting full-strength acids on your own is also not recommended. Hydrochloric acid fumes are toxic. Just the slightest whiff will burn your nose. Most acids are already sold in diluted form either online or at chemical or building supply stores.  Read and follow the instructions and safety guidelines for any acid you purchase.

Supplies

Using vinegar is recommended. It's acidic enough for testing purposes and has less potential for danger.

If you're diluting acid, you'll need the following:

• A glass jar with a glass lid to mix your acid. (Metal lids will dissolve from the vapors).
• Water.
• Plastic gloves. Wear these while handling acid.

Remember to label all containers clearly and store them somewhere safe, out of the reach of children and pets.

Procedures

Always work in a well-ventilated area when handling acids.

1. Place a very small drop of vinegar, no larger than a pinhead, on an inconspicuous spot on your specimen. For faceted gems, this is usually near the girdle.
2. Observe the reaction. If the test material is a carbonate, the acid will effervesce.
3. Since you're using such a small amount, low-power magnification helps to see the results.
4. The reaction happens quickly, so wash the stone under running water as soon as possible.

Acid Testing For Lapis Lazuli

You can also use acid testing to distinguish one type of imitation lapis lazuli (not the glass or plastic types). The white spots in lapis are calcite. Lightly colored lapis is a result of fine particles of calcite throughout the lazurite. To differentiate between natural and imitation lapis, perform the acid test above. If the stone is natural, you'll detect a rotten egg smell, hydrogen sulphide. It may effervesce if enough calcite is present. In the imitation version, you'll also detect the rotten egg smell but no effervescence. The acid will also leave a white spot.

What Is The Difference Between Gemology And Mineralogy For Acid Testing?

The difference between gemology and mineralogy when it comes to acid testing lies in the focus, materials, and the practicality of the test. As mentioned above, acid testing is primarily used to determine a material's reactivity to acids, which helps identify certain minerals and gemstones.

Acid testing is rarely used in gemology because it is a destructive test and risks damaging or etching the surface of a valuable gemstone. However, it can be used for specific cases where identification is difficult, such as carbonate gemstones. Acid testing can identify organic gems like calcite, aragonite, and pearls. For example, though we don't recommend using acid testing on pearls, they do react to dilute hydrochloric acid (HCl) by fizzing (producing CO₂ bubbles). Malachite effervesces slightly with acid due to its carbonate content. In this way, acid testing can help distinguish genuine gemstones from imitations or synthetics that react differently to acids, but non-destructive methods like spectroscopy and refractometry are almost always preferred.

By contrast, acid testing is a common diagnostic tool in mineralogy for identifying carbonate minerals and determining mineral composition. Since mineralogists often work with raw, unpolished specimens, surface damage is less of a concern.

Acid testing can identify minerals containing carbonate ions (CO₃²⁻) because there is a reaction to the acid that releases carbon dioxide (CO₂). For example, dolomite reacts weakly with cold HCl but more vigorously when powdered or heated. In mineralogy, acid testing can also help distinguish between minerals that appear similar visually but have different chemical compositions.

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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