Step 2: Advanced Gemology

Lesson 2

The optical properties of gems attract our eyes and can give a cut stone a unique character. However, gemstone physical properties are also important. They determine a gem’s durability, guide faceters when they cut and set, and help us gemologists identify gemstones.

Specific Gravity

Specific gravity (SG) or density refers to how much something weighs in relation to its size. For example, a cubic centimeter of steel is much heavier than a cubic centimeter of styrofoam. SG is expressed as the relationship to an equivalent volume of water. For example, a gem with a specific gravity of 3 weighs three times as much as the same amount of water.

In gemology, the terms specific gravity and density are interchangeable.

Specific Gravity Values of Popular Gemstones

Gem SG
Garnet 3.40 – 4.25
Corundum 3.97 – 4.03
Topaz 3.53 – 3.56
Diamond 3.51 – 3.53
Peridot 3.27 – 3.48
Tourmaline 2.84 – 3.10
Beryl 2.66 – 2.80
Quartz 2.63 – 2.68
Opal 1.99 – 2.25
Amber 1.05 – 1.096

As you can see from the chart, few gems share the same SG range. This makes SG one of the most useful…