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History
The principle of specific gravity was first described by Archimedes around 250 B.C., according to the NDT Resource Center. Archimedes had been instructed by the king to deduce if the gold in the king's crown was pure or mixed with cheaper silver. Archimedes hit upon the idea of weighing the crown, then comparing the amount of water it displaced with a sample of pure gold of the same weight.
Definition
Specific gravity, or SG, is a unitless measure. It is derived from the density of the mineral divided by the density of water. For example, if a mineral has a specific gravity of 2, then its density is twice that of water.
Examples
The minerals calcite, feldspar and quartz, which make up most of the Earth's crust, have an SG of around 2.75, which is considered average. Many familiar minerals have a similar SG, but some are much lighter or heavier. For example, borax is considered light with an SG of 1.7. Sulfur (2.0 to 2.1) is about average. Hematite (4.9 to 5.3) is heavy, but gold is heavier still, with an SG of 19.3.