Specific Gravity Testing Part 1: Definitions

Before getting into the details of techniques and equipment, some terms must be defined.

What is Specific Gravity?

Specific gravity is also known as relative density or just density. Density is mass per unit of volume or the ratio of the amount of matter in an object to its volume. For example, if you had a piece of plastic and a piece of lead the same size, the lead would be much heavier. That is an illustration of density, how much something weighs in relationship to its size.

Specific gravity is a case of relative density, which is defined as the ratio of the density of one substance to that of another. For gemologists, specific gravity is the relationship of the weight of a gemstone to an equivalent volume of water. When you read that a gem has a specific gravity of 3, it means that the gem weighs three times as much as the same amount of water.

The distinction between density and specific gravity is important in some sciences, but in gemology the terms are interchangeable. For example, it’s acceptable to say that a gem has either the…

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