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Frequently Asked Questions (FAQ) / Specific Gravity for Gems

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 that 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 values, various measuring devices are used. Rough approximations of the specific gravity of lighter stones can be made by using a series of liquids with known specific gravity values. For example, if a stone floats in a liquid with a specific gravity of 4 and sinks in a liquid with a specific gravity of 3, the specific gravity of the stone must lie between these values.

See our specific gravity chart for density ratings of the different types of gemstones.

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  • First Published: February-20-2006
  • Last Updated: November-11-2014
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Size and Weight

Gems are always measured in Millimeter (mm)

Dimensions are given as;
length x width x depth,
except for round stones which are;
diameter x depth

Select gems by size, not by weight!
Gem varieties vary in density, so carat weight is not a good indication of size

Note: 1ct = 0.2g

Size Comparison Chart