Step 3: Practical Gemology

Lesson 10

Getting accurate readings when testing gemstone specific gravity is critical. If you make an error of 0.01 carats weighing a 10-carat gem, it will only affect the results by 0.03. That’s not significant. However, on a 1-carat stone, a 0.01 weight error will make a 0.08 difference in your specific gravity calculations. That’s enough to misidentify some stones. On a half-carat stone, the same error changes the specific gravity reading by 0.13. With smaller gems, errors lead to greater differences.

To help ensure maximum accuracy, follow these detailed procedures for refining your specific gravity testing techniques.

Choosing Liquids for Specific Gravity Testing

Distilled Water

You can use any liquid for your tests. Water is used frequently. However, never use tap water. Its density or specific gravity isn’t constant and is always higher than distilled water. Note that the temperature correction table below refers to pure, distilled water, which you can find in any supermarket. If you use distilled water, you can minimize the surface tension by adding a bit of liquid soap or detergent. Two or three drops in a quart or liter won’t make a significant difference in the density…