CHAPTER III.
PHYSICAL PROPERTIES.
A—Crystallise Structure.
Before proceeding to the study of precious stones as individual gems, certain physical properties common to all must be discussed, in order to bring the gems into separate classes, not only because of some chemical uniformity, but also because of the unity which exists between their physical formation and properties.
The
first consideration, therefore, may advisedly be that of their
crystals, since their crystalline structure forms a ready means for the
classification of stones, and indeed for that of a multitudinous
variety of substances.
It
is one of the many marvellous phenomena of nature that mineral, as well
as many vegetable and animal substances, on entering into a state of
solidity, take upon themselves a definite form called a crystal. These
crystals build themselves round an axis or axes with wonderful
regularity, and it has been found, speaking broadly, that the same
substance gives the same crystal, no matter how its character may be
altered by colour or other means. Even when mixed with other
crystallisable substances, the resulting crystals may partake of the
two varieties and become a sort of composite, yet to the physicist they
are read like an open book, and when
