Since water's density is one gram per cubic centimeter (under standard conditions), relative density has the same value as density expressed in grams per cubic centimeter.

Specific gravity is often used by geologists and mineralogists to help determine the mineral content of a rock or other sample. Gemologists use it as an aid in the identification of gemstones. The reason that specific gravity is measured in terms of the density of water is because that that is the easiest way to measure it in the field. Basically, density is defined as the mass of a sample divided by its volume. With an irregularly shaped rock, the volume can be very difficult to accurately measure. The most accurate way is to put it in a water-filled graduated cylinder and see how much water it displaced. Another method is to suspend the sample from a spring scale and weigh it under water.

Specific gravity can be measured accurately by use of sensitive laboratory equipment such as hydrostatic balance. A fair estimate of specific gravity can be obtained by use of a rigged balance system. A mineral must be weighed first dry, D, and then weighed, W, while emersed in a liquid, usually water, then the specific gravity can be obtained by the following formula: SG = D/([D-W]L), where L is the density of the liquid, if water is used, L=1, so that the formula is simply D/(D-W). The mineral being tested must be homogenous and not contain other minerals. It is best to use single small crystals that are not attached to a host matrix.