Specific gravity is defined, simply, as the density of a gem, mineral or other material, relative to water.
For example, the specific gravity of diamond is 3.52. That means that a diamond is 3.52 times heavier than an equal volume of water.
The Specific Gravity of a gem can be diagnostic in determining its identity!
The calculation can be done directly, if one has a scale and a hydrostatic weighing device. (A hydrostatic weighing device is fairly rudimentary and can easily be constructed out of wire and spare parts laying around most households, or inexpensively purchsed from several sources.)
For Gemology, we write the equation as follows:
The first thing we do, is weigh the gemstone on an appropriate scale that will give us an accurate carat weight.
Secondly, the gem must be weighed suspended in water.
Simply, plug these values into the above equation, and voila, you have the gem's specific gravity.
Another common way, to determine the relative specific gravity of a gem is to use heavy liquids.
Heavy liquid kits are available from several sources specializing in tools for the gemologist.