Physical Properties of Minerals Part 2 With Simple Illustration.

Specific gravity

Specific gravity (SG) is a measure of the density of a substance relative to that of water, and determines how dense a gemstone is. It is measured as the ratio of the mass of the substance and the mass of an equal volume of water—so a mineral with an SG of 2 is twice as heavy as water. Specific gravity can be determined using specialized balances or liquids that allow minerals below a given SG to float and those above it to sink. However, experts can often gauge the specific gravity of a gem purely by its heft.


Fracture is another way of describing how a mineral breaks. In fracture, however, breakage takes place across the mineral’s atomic planes, rather than along them, as it does in cleavage, because there are no obvious planes of fracture. Distinctive fracture may help with identification.


Cleavage is the property of a mineral that causes it to break along its atomic layers, where the forces bonding its atoms are the weakest. Some gems have cleavages in several directions, some of which may be very easy to trigger, meaning that the gem can be easily broken if sharply knocked. Because they follow the atomic planes, cleavage surfaces are often smooth.

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