Specific gravity (SG), or relative density, is the weight of a gem compared to an equal volume of water. Most gems fall between 2.5 and 4.5. Gemologists determine SG using hydrostatic weighing (weighing in air and water) or immersion in heavy liquids calibrated to known densities.
This test quickly separates dense stones like garnet (SG 3.5–4.3) from lighter quartz (SG 2.65). Combined with refractive index and microscopic examination, SG provides high-confidence identification. For instance, it helps distinguish natural from synthetic material when growth features are ambiguous. SG also offers clues about chemical composition, as heavier elements increase density.