The Specific Gravity ($G_s$) of rock is the ratio of the density of the intact rock mineral solids to the density of distilled water. This dimensionless value is a critical index property used to identify rock mineralogy and assess the material's overall physical competence.
Specific gravity is determined using the Archimedes Principle. Representative rock samples are thoroughly cleaned to remove surface debris and then weighed in two states: dry in air, and suspended in water.
By measuring the buoyant force exerted by the water, we calculate the exact volume of the rock solids. A higher specific gravity (typically 2.6 to 2.9 for common rocks like granite or basalt) suggests a dense mineral structure with high strength. Deviations from these norms often signal internal fractures or chemical weathering, alerting engineers to potential stability risks.