The following brief notes as to the physical or mechanical properties of minerals not already discussed or described must suffice.
Form.—The forms of crystals are all referable to one or other of these six crystallographic systems, (1) The cubic or monometric; (2) the pyramidal, dimetric, or tetragonal; (3) the rhombohedral or hexagonal; (4) the prismatic, trimetric, or orthorhombic; (5) the oblique or monoclinic; (6) the triclinic or anorthic.
Structure.—The mode of mechanical aggregation or intimate texture of minerals may often be learnt by disruption of the mass, or by splitting or cleaving it. Structure is often crystalline, laminar, fibrous, or columnar. Fractured, not cleaved, surfaces are less instructive—they may be conchoidal, uneven, splintery, or hackly.
Transparency.—For want of a more comprehensive term, the various degrees of resistance to the transmission of light through minerals are included under this title. The degrees are five:
- Transparent—when objects can be seen distinctly.
- Semi-transparent—when objects can be seen dimly.
- Translucent—when light, not objects, can be seen.
- Subtranslucent—when light is transmitted through thin splinters.
- Opaque—when light is not transmitted.
Lustre. This character, although it needs some practice to discern it accurately, is of importance as an element not merely of the, beauty but also in the discrimination of precious stones. The terms employed to designate its various qualities are these:
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Metallic, as on pyrites.
Adamantine, as on„ diamond.
Resinous, as on„ garnet.
Vitreous, as on„ emerald.
Waxy, as on„ turquoise.
Pearly, as on„ moonstone.
Silky, as on„ crocidolite.
