Specific Gravity
Definition
The ratio of a mineral's weight compared to an equal volume of water. It indicates how dense a mineral is.
Example
Gold has a specific gravity of about 19.3, making it much heavier than most minerals.
The Density of Gems
Specific Gravity (SG) is a critical diagnostic tool for gemologists. It describes the density of a material relative to water. Because different minerals have different chemical compositions and crystal structures, they pack atoms differently. For example, a diamond (SG 3.52) feels much heavier than a piece of quartz (SG 2.65) of the exact same size. Gold is incredibly dense (SG 19.3), which is why panning works—the heavy gold sinks while the lighter sand washes away.
How to Measure SG
The most common field method is "hydrostatic weighing." You weigh the dry stone in air, and then weigh it suspended in water. The difference between the two weights allows you to calculate the volume and thus the SG. Gemologists also use "heavy liquids"—fluids calibrated to specific densities. If a stone floats in a liquid of SG 3.0, its SG is lower than 3.0; if it sinks, it is higher.