The specific gravity of a mineral is a measurement of its density. To find it, measure the weight of your mineral. Then suspend it underwater from a scale and measure its weight submerged. The specific gravity is then calculated as:

                            weight in air
specific gravity = ---------------------------------
                   (weight in air - weight in water)

Warning: Minerals with a large amount of iron (e.g., hematite, magnetite, tiger-eye quartz) or which are very soluble (e.g., halite, niter, sylvite) should never be exposed to water! Doing so will damage the minerals.