Specific Gravity, also known as SG, is a measurement that determines the density of
minerals. Two minerals may be the same size, but their weight may be very different. The
specific gravity of a mineral determines how heavy it is by its relative weight to water.
The specific gravity value is expressed upon how much greater the weight of the mineral is
to an equal amount of water. Water has a specific gravity of 1.0. If a mineral has a
specific gravity of 2.7, it is 2.7 times heavier than water. Minerals with a specific
gravity under 2 are considered light, between 2 and 4.5 average, and greater than 4.5
heavy. Most minerals with a metallic luster
are heavy. The specific gravity may slightly vary within a mineral because of impurities present in the minerals structure.
How to use specific gravity as an
Scientists measure specific gravity with expensive
laboratory tools, such as a hydrostatic balance. These tools are not used by
regular mineral collectors, and the procedure for testing with them will not be mentioned
here. There are other methods to determine specific gravity, such as using water
displacement, but this is a complicated procedure that can provide inaccurate results.
Instead of actual testing for a specific gravity value, the heft of a specimen often
provides sufficient results. It is easy to notice a very light specimen, an average
specimen, and a heavy specimen.
How to test using specific gravity
Testing a mineral for a specific gravity value is a
complicated procedure. For the layman, it is done by water displacement
and requires a beaker and a scale. The weight of the
beaker is taken and written down, as well as the weight of the specimen. The beaker is
partially filled up with water, and the level of the water is noted. The mineral is put
into the beaker with water, and the water level rises. The difference in the amount of
water before the specimen was put in and after it was put in is noted. The mineral is
taken out, and the water is spilled out. Then the beaker is filled with the amount of
water that the specimen displaced and measured. The difference in weight of the beaker
when it was empty and the current measurement (the beaker with the displaced water) is the
weight of the displaced water. The weight of the displaced water has the same volume as
the specimen, but a different mass. The weight of the specimen is divided by the weight of
the displaced water, and that number attained is the specific gravity of that specimen.
This test cannot be conducted for an embedded mineral, but
only for a single crystal or mass, for obvious reasons.