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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:
Weight of Mineral in Air
SG =
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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:
Weight of Mineral in Air
SG = ---------------------------------------------------------
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:
Weight of Mineral (in grams)
SG = ------------------------------------------------
Volume of Mineral (in cubic cm)
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 to overview of
Specific Gravity
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