# About Specific Gravity

#### What is specific gravity?

It is the density of a substance relative to water.  As an example, when we say that the specific gravity of quartz is 2.65, we mean the weight of quartz is 2.65 times that of an equal volume of water.  Their are a number of ways to write the equation for the specific gravity (SG) of a mineral.  Here is the most common:

Specific Gravity  =                     Weight of Mineral in Air
____________________________________________

(Weight of Mineral in Air  -  Weight of Mineral in Water)

So from this equation, you need only first weigh the mineral in air (which means the same as "weighing it"), then suspend the mineral in water for the second weighing.  Since Archimedes discovered that the weight of the mineral in air minus its weight in water is equivalent to the weight of the water displaced by the mineral, the equation can also be written as follows:

Specific Gravity  =                     Weight of Mineral in Air
____________________________________________

Weight of Water displaced by Mineral

And since the weight of 1 cubic centimeter of water equals 1 gram, the equation can then also be written as follows:

Specific Gravity  =                     Weight of Mineral (in Grams)
____________________________________________

Volume of Mineral (in Cubic Centimeters)

#### Why measure the specific gravity of a mineral?

Because specific gravity is one of the most important, constant, and quantifiable properties of a gem or mineral, and in your effort to identify an unknown stone, the specific gravity, if known, can narrow down your search to just a handful of possibilities.

#### Must my mineral specimen be pure?

Yes and No.  No, because you can theoretically measure the specific gravity of any solid using these devices, whether it be a chunk of plastic, a rock containing a varying amount of minerals, or a pure mineral itself in the form of a crystal.  But yes, your mineral specimen must be pure if you wish to compare your measurement to standards of specific gravity for those minerals, which standards were developed using pure mineral specimens.  In summary, the more pure your specimen, the more accurate will be your specific gravity measurement.

Back todevices

Also see