Specific Gravity Testing Part 1: Definitions


Step 3: Practical Gemology

Lesson 7

Specific gravity testing is a technique gemologists use to differentiate and identify gems with similar appearances. For example, although a garnet and ruby may look alike, each gem species has a unique specific gravity.

This is the first of a 5-part series of articles on specific gravity testing. 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 …

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