Specific gravity (SG) is defined as:

Specific gravity (SG) is fundamentally understood as the ratio of a gemstone's weight to the weight of an equivalent volume of water. This measurement helps in determining the density of the gem material relative to water, which has a specific gravity of 1. By assessing SG, gemologists can identify gemstones, as different materials exhibit unique density characteristics that serve as identifying markers.

For example, if a gemstone has a specific gravity of 3, it means that it is three times as heavy as an equal volume of water. This property is crucial not only for identification but also for understanding how a gemstone will behave with respect to light, refraction, and overall appearance.

The other options such as surface area, color intensity, and hardness refer to different properties of gemstones that are not related to their specific gravity. Surface area concerns the physical dimensions and shape of a stone, color intensity pertains to the vibrancy and depth of the gem's color, and hardness is a measure of a material's resistance to scratching. None of these factors directly convey the mass-to-volume ratio that defines specific gravity.