Specific Gravity is the ratio of the weight of a substance to the weight of an equal volume of water. Most gem species have a consistent specific gravity throughout. The differences in specific gravity between gem species can be used in gem identification. In order to determine a gem’s specific gravity, it is necessary to weigh the gem in air and again in water. The water measurement will be lower because the gem loses weight equal to the weight of the water it displaces. Simple math, dividing the weight in air by the weight in water, yields the gem’s specific gravity.