Specific gravity (SG) is the density of a stone relative to the density of water. The specific gravity of water is one (1). If a stone has a SG is 3.95, then stone has a density that is 3.95 times greater than the density of water. SG is determined by dividing the stone's weight in air by the weight of the water is displaces when it is immersed in water. Because it does not require a polished surface, SG is one of the best tests for rough stones. You can perform the test in the field and with relatively inexpensive equipment. The following directions are for the PrettyRock.com Specific Gravity Travel Kit. The kit fits stones up to about 50 ct or approximately 25 x 25 x 15mm and has reasonable accuracy up to .33 ct. You will need the kit, a calculator, and something to take notes on.