Specific gravity is a way to express the relative density of a gemstone. It is measured as the ratio of the density of the gemstone to the density of water. It is expressed as a number which indicates how much heavier the gemstone is compared to an equal volume of water.

Scientifically, specific gravity is defined as a ratio of the mass of a given material to the mass of an equal volume of water at 4 degrees centigrade. Most gemstone substances are two to four times denser than an equal volume of water. Specific gravities are expressed in decimal numbers, for example, 4.00 for corundum, 3.52 for diamond , and 2.72 for quartz. Zircon, one of the densest of all gemstones, may have a specific gravity as high as 4.73!

Specific gravity is an important tool in gemstone identification. But it is useful for the gem buyer as well as the gemologist. A 1 carat stone with higher specific gravity will be smaller than a 1 carat stone with lower specific gravity. So don't be surprised when the sapphire you buy is smaller than your diamond. That's because sapphire has a higher specific gravity. On the other hand, the 5 carat tourmaline you buy might be larger than you expected, because tourmaline has a fairly low specific gravity.

The following chart lists about 60 varieties of gemstones, sorted by specific gravity in descending order: