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PRECIOUS STONES.
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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."