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Electrical properties
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The
following electrical properties are possessed by certain gem-stones,
e.g. frictional electricity, pyroelectricity and piezo-electricity.
Frictional electricity.—Diamond, tourmaline, topaz are positively electrified when rubbed with a cloth, whereas amber develops a negative charge.
Pyro-electricity.—Tourmaline
is an excellent example of a pyroelectric mineral. The crystal when
heated develops, positive and negative charges on its opposite ends.
The test is done by gentle heating of the crystal bringing it into
contact with a conducting medium, when the electricity developed during
the heating is discharged. Quartz also exhibits pyroelectricity.
Piezo-electricity.—Electrical
charges are developed in certain minerals by application of pressure,
e.g. quartz and tourmaline. Properly oriented sections of quartz are
used for frequency control in electric and radio apparatus.
Chapter VI PHYSICAL PROPERTIES
SOME
of the physical properties of gem-stones have already been dealt with
under colour, lustre, etc. Now we may proceed to consider the structure
and other related properties of gem-stones.
The structure of minerals may be (a) amorphous with no definite structure, e.g. opal and glass; (b) cryptocrystalline
whose structure is revealed only by the microscope, e.g. agate and
flint, and (c) crystalline aggregates, e.g. coarse marble. In a study
of gem-stones, we are concerned with crystalline aggregates and
crystals, whose internal structure is expressed by their external form
or faces and crystals are more valuable for gems, because they are
better formed than the aggregates and are distinct.
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