structure on which depend so many of the properties already mentioned.
In
relation to precious atones, crystallography is chiefly of importance
in the case of uncut gems, or in the cutting of a stone; but a
knowledge of the properties dependent on the internal structure aids
very greatly in the identification of a cut specimen (cf. Physical Properties Dependent on Light).
When
a mineral only shows the internal structure without any definite
external crystal forms it is said to be "crystalline." When the
external form is developed the internal arrangement of the molecules is
always present, and the mineral is then said to be "crystallised."
Crystals
deposited from an ordinary aqueous solution are usually found to have
started their growth from some preexisting solid body; the point from
which this growth commences is known as the point of attachment. Many
of the beautiful groups of Rock Crystal from the Alps show this very
clearly. Such crystals are necessarily imperfect in outline at this
point.
When
crystals are deposited during the cooling of rocks undergoing
hydro-thermal metaniorpbisra, two modes of occurrence are to be
noticed. If the mineral in question is one which for some reason or
another crystallises out before the majority of its associated
minerals, its crystals are very likely to be perfectly developed all
round—they have the characteristic external shape of the crystals of
that mineral; they are then said to be " idiomorphic." Crystals which
during their deposition are, so to speak, crowded against other
crystals, may mutually compress one another so that in a large part
their external form is not the characteristic one; in such a case they
are " allotriomorpbic."
