SG =(weight of mineral in air)/(the weight of an equal volume of water)
1) Weigh the unknown specimen while it is dry.
2) Then using a special scale designed for this purpose, weigh the specimen while it is suspended in water. This is done because the amount of weight that is lost when a mineral is weighed in water is the same as the weight of the amount of water displaced by the mineral (volume of the mineral specimen). Typically it is hard to know the exact volume of a mineral, so weighing in water is the easiest method.
3) Finally, the mineralogist divides the loss of weight into the dry weight. This gives the specific gravity of the unknown mineral specimen.
For an example, we can use a sample of gold as follows:
Dry weight: 50 gms.
Weight when suspended in water: 47.368 gm.
Loss of weight: 2.632 gm.
Dry weight divided by loss equals specific gravity ,about: 19
In fact, the specific gravity of native gold varies between 19
and 20. This results in part from impurities or even hollow spaces (bubbles)
that may be present in varying quantities in different specimens of the
mineral. Most minerals have a range of potential specific gravities. In
some the variation may be as much as 25%. Therefore, it is wise to choose
as pure a mineral specimen as possible when testing the physical properties
of an unknown mineral specimen. Different varieties of the same mineral
may have different SGs, for example garnet group minerals or tourmaline
group minerals depending on the major element composition of the varieties.