Specific gravity is a measure of the density of a material. It compares the density of the material to the density of water. Density is a characteristic property of any material found by dividing the mass of the material by its volume.
Specific gravity has no units.
The material can be a mineral. Specific gravity can be used as a way to tell the difference between minerals and to help identify them. For example, the specific gravity of diamond is 3.5, platinum is greater than 20, and amber is about 1.1. The rocks found in the outer surface of the earth's crust are composed mostly of the minerals quartz, calcite, and feldspar. These three minerals have specific gravities around 2.75.