Specific gravity is a measurement that determines the density of minerals. It is a ratio of the minerals mass to the mass of an equal volume of water. Different minerals have different densities and thus different specific gravities. The specific gravity of a mineral determines how heavy it is by its relative weight to water. Minerals with a specific gravity under 2 are considered light, between 2 and 4.5 average, and greater than 4.5 heavy.
The use of specific gravity is significant in the differentiation of minerals because it is one of the most important, constant, and quantifiable properties of a gem or mineral. It can help confirm the identity of minerals, especially when samples of the same mineral may appear quite different. Specific gravity can also be used to distinguish pure gold from 14K gold or pyrite. In addition, the specific gravity of a mineral can narrow down the search for an unknown stone to just a handful of possibilities.
To measure specific gravity, mineral specimens are weighed in air and then suspended in water for the second weighing. The weight of the mineral in air minus its weight in water is equivalent to the weight of the water displaced by the mineral.
