|
18
PRECIOUS STONES.
|
||
|
|
||
|
time — a quality denominated phosphorescence. The diamond,
in some of its varieties, naturally possesses this attribute to a certain extent. Electricity. —The property of attracting or repelling certain
substances inheres in some bodies, while in others, naturallv non-electric, it may be excited by heat, friction, or pressure. Precious stones are more or less electrical either positively or negatively, while some species are positive at one end of the crystal and negative at the other, as the tourmaline. Another important quality in precious stones for the purposes of jewelry is that of splitting in definite directions,
which is called cleavage. Isomorphism is a term applied to crystallized compounds
formed of substances differing essentially in their nature, but appearing to be identical. Crystallization is the property which certain substances possess of solidifying in regular shapes. The name crystal, "ice,"
was given to quartz by the ancients, from the belief that this mineral was solidified water. Sometimes the term is inaccurately applied to flint glass, which is not crystalline in nature. When bodies cool in solid mass, as in the case of some precious stones, they are said to be amorphous. "The process of crystallization," says Professor Cook, "is
one of the most striking phenomena in the whole range of experimental science. Beautiful, symmetrical forms shape themselves in an instant, out of a liquid mass, revealing an architectural power, in what we call lifeless matter, whose existence and controlling influence but few have probably realized." The substance at the time of crystallization is thought to
have been in a state of fusion, gas, vapor, or solution. In the formation of crystals, a different law predominates from that which controls organized beings ; that is, they grow externally, |
||
|
|
||