Specific Gravity
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Definition
The ratio of a gemstone's weight to the weight of an equal volume of water at 4°C, providing a density measurement that is unique to each gem species.
Detailed Explanation
Specific gravity (SG) is determined by weighing a stone in air and then in water and applying Archimedes' principle. Diamond has an SG of 3.52, sapphire 4.00, and emerald approximately 2.72. Because SG is a characteristic constant for each gem species, it is a key identification tool. For example, a suspected ruby can be distinguished from red spinel (SG 3.60) versus ruby (SG 4.00) through hydrostatic weighing. Heavy liquids with known densities are sometimes used to rapidly sort gem rough.