What is "specific gravity" for gemstones? What does it measure?

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 the 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 gemstones, may have a specific gravity as high as 4.73!

For exact determinations various weighing devices are used, but rough approximations of the specific gravity of lighter stones can be made by means of a series of liquids of known specific gravity. If the stone will float in a liquid having a specific gravity of 4 and sink in a liquid with a specific gravity of 3, the specific gravity of the stone must lie between these limits and be approximately 3.5.

See our Specific Gravity chart for density ratings of the different types of gemstones.

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