Gemstone Physical Properties
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.
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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 gemstone physical properties you can have for identification. However, measuring specific gravity is difficult and time consuming. Thus, you normally only conduct this test when necessary. (See “The Art and Science of Identifying Gemstones” for new identification procedures that don’t typically require SG testing).
Using Specific Gravity to Estimate Gem Weight
When you’re buying and selling gems, knowing the SG of a gem can help you make the right choice.
For…
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.”
International Gem Society
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