Specific Gravity is a measure of the density of a mineral.
At times it is such a useful property that it is the only way to
distinguish some minerals without laboratory or optical techniques.
(pictured) can easily be distinguished from "fool's gold" by specific gravity alone, although there are many other ways.
Specific gravity is a unitless measure, because it is derived from
the density of the mineral divided by the density of water and
thus all units cancel. However, since water's density equals 1
gram per cubic centimeter (at specific conditions), then a mineral's
specific gravity would also correspond to a mineral's density
as expressed in grams per cubic centimeter.
Specific gravity, or SG, as already stated compares the density of a mineral to
the density of water. If a mineral has a SG of 2, then it is twice
as dense as water. If a mineral has a SG of 3 then it is three
times as dense as water and so forth. However, comparing a mineral
to water is not practical and is not really helpful. It is easier
to consider what is the SG of a typical mineral and compare minerals
The Earth's crust, where we are most likely to collect minerals
from, is composed mostly of the minerals quartz,
These minerals have SG's around 2.75 and that is close to the average
SG of the rocks on the outer surface of the Earth's crust. Therefore,
the SG of most rocks that people would pick up and be familiar
with, will have a specific gravity of approximately 2.75. To use
specific gravity, hold a mineral of unknown SG in one hand and in the other hand
a mineral of known SG preferably one near the average of 2.75 and of
the same size as the unknown mineral; then
compare. How does the unknown mineral compare?
By convention, comparisons of SG are split between
luster categories: non-metallic and metallic.
Non-metallic minerals tend to be of a low density and we naturally expect them to be so.
So, when one is surprisingly heavy, it becomes very diagnostic.
The following unscientific scale is used for specific gravity
comparisons with average crustal non-metallic minerals
(such as quartz,
- 1 -- 2 . . . . . . very light
- 2 -- 2.5 . . . . light
- 2.5 -- 3 . . . . average
- 3 -- 3.5 . . . . slightly above average
- 3.5 -- 4 . . . . above average
- 4 -- 5 . . . . . . heavy
- 5 -- 7 . . . . . . very heavy
- 7 -- 10 . . . . . extremely heavy
10+ . . . . . . beyond being extremely heavy
Metallic minerals, which are usually composed of heavy elements
such as iron, lead or silver, have a higher SG than their translucent
cousins. Therefore it is helpful to have a scale comparing metallic
minerals with an average specific gravity of around 4.5. The following
scale is intended as a comparison of only metallic minerals.
- 1 -- 3 . . . . . . very light
- 3 -- 4 . . . . . . light
- 4 -- 5 . . . . . . average
- 5 -- 6 . . . . . . slightly above average
- 6 -- 7 . . . . . . above average
- 7 -- 10 . . . . . heavy
- 10 -- 20 . . . . very heavy even for metallic minerals
- 20+ . . . . . . . must be platinum
In some minerals a solid solution series exists in which substitution
of one element occurs with another element in the chemistry of
the mineral. Such a situation is found in the mineral
(Mg, Fe)2SiO4 .
When near pure Mg2SiO4,
olivine has a SG of approximately 3.3 and when near pure
olivine has a SG of approximately 4.2. Most olivine is a mixture
and the percentage of iron can be determined with some certainty
by using the SG.
Specific gravity can be measured accurately by use of sensitive
laboratory equipment. For the average rockhound a good estimate
of SG can be obtained by use of a rigged balance system. If the
mineral can be weighed first dry, D, and then weighed,
emersed in a liquid, usually water, then the
SG can be obtained
by the following formula: SG = D/([D-W]L), where
L is the density
of the liquid, if water is used, L=1, so that the formula is simply D/(D-W).
The mineral being tested
must be homogenous and not contain other minerals. It is best
to test single small crystals that are not attached to a host
Technical Crystal Habits
Descriptive Crystal Habits
Index of Refraction
Reaction to acids