Chapter IV
FORMS OF CRYSTALS
IT
has been mentioned that gem-stones are minerals. A mineral is defined
as a substance occurring in nature with a characteristic chemical
composition, and usually possessing a definite crystalline structure.
When crystalline it has also external geometrical forms or outlines.
Definite crystalline structure means that the structural units of a
mineral, i.e. groups of matter have a definite and an orderly
arrangement, which is characteristic of each particular mineral. Most
gem minerals exhibit external geometrical forms or outlines. These
forms are known as crystals. The diamond, ruby and sapphire and emerald
occur as crystals.
Crystal
structure and X-rays has been a subject of special study which is not
included in our study of gem-stones. Each kind of gem-stone has its
characteristic crystalline form and may occur in well-formed crystals.
If one looks at the crystalline faces, one can often say at a glance to
which mineral the crystal belongs. Even water worn crystals could be
identified by their general shape. Thus an elementary study of
crystallography is of value to the student of gemmology, who will find
it helpful to identify easily uncut material. Moreover, a knowledge of
the crystallographic properties is essential for the efficient cutting
of gems.
12
