Specific Gravity is a measure of the density of a mineral. At times it is such a useful property that it is the only way to distinguish some minerals without laboratory or optical techniques. Gold (pictured) can easily be distinguished from "fool's gold" by specific gravity alone, although there are many other ways. Specific gravity is a unitless measure, because it is derived from the density of the mineral divided by the density of water and thus all units cancel. However, since water's density equals 1 gram per cubic centimeter (at specific conditions), then a mineral's specific gravity would also correspond to a mineral's density as expressed in grams per cubic centimeter.
The Earth's crust, where we are most likely to collect minerals from, is composed mostly of the minerals quartz, calcite and feldspar. These minerals have SG's around 2.75 and that is close to the average SG of the rocks on the outer surface of the Earth's crust. Therefore, the SG of most rocks that people would pick up and be familiar with, will have a specific gravity of approximately 2.75. To use specific gravity, hold a mineral of unknown SG in one hand and in the other hand a mineral of known SG preferably one near the average of 2.75 and of the same size as the unknown mineral; then compare. How does the unknown mineral compare? By convention, comparisons of SG are split between luster categories: non-metallic and metallic. Non-metallic minerals tend to be of a low density and we naturally expect them to be so. So, when one is surprisingly heavy, it becomes very diagnostic.
The following unscientific scale is used for specific gravity
comparisons with average crustal non-metallic minerals
(such as quartz,
Metallic minerals, which are usually composed of heavy elements
such as iron, lead or silver, have a higher SG than their translucent
cousins. Therefore it is helpful to have a scale comparing metallic
minerals with an average specific gravity of around 4.5. The following
scale is intended as a comparison of only metallic minerals.
In some minerals a solid solution series exists in which substitution of one element occurs with another element in the chemistry of the mineral. Such a situation is found in the mineral olivine, (Mg, Fe)2SiO4 . When near pure Mg2SiO4, olivine has a SG of approximately 3.3 and when near pure Fe2 SiO4, olivine has a SG of approximately 4.2. Most olivine is a mixture and the percentage of iron can be determined with some certainty by using the SG.
Specific gravity can be measured accurately by use of sensitive laboratory equipment. For the average rockhound a good estimate of SG can be obtained by use of a rigged balance system. If the mineral can be weighed first dry, D, and then weighed, W, while emersed in a liquid, usually water, then the SG 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 test single small crystals that are not attached to a host rock.