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Ch. 8: Corundum

Ch. 8: Corundum Page of 311 Ch. 8: Corundum Text size:minus plus Restore normal size   Mail page  Print this page
186
PRECIOUS STONES.
Phosphorescence is well marked in some specimens, and is well seen in a darkened room when the stone is carefully heated. Fluorescence is seen in some kinds, as the Oriental Emerald from Siam.
It is quite infusible before the ordinary blowpipe. On rubbing with a dry cloth it shows a surface charge of positive electricity.
The specific gravity is high, varying from 3'93 to 4-08, so that it readily sinks in a saturated solution of iodine and iodoform in methylene iodide.
The fracture is conchoidal to uneven, and the mineral is
brittle. Its hardness is marked, being excelled amongst minerals by the Diamond alone. It is the Standard 9 of Mohs' scale.
True cleavage is absent, but from a lamellar twinning parallel to the basal plane and to the unit rhombohedron there are more or less well marked parting planes parallel to these faces in such twinned specimens, and this simulates cleavage closely.
Crystalline form. Corundum belongs to the rhombo-hedral division of the hexagonal system, but as in the case of Quartz, the rhombohedral character is not always very apparent, the general form of the crystal being often that of a doubly terminated and rather acute hexagonal pyramid, not infrequently truncated by a basal plane (Fig. 14). Oscillations of pyramid forms between the basal plane and the prism faces causes a characteristic wavy outline in many crystals, and in nearly all cases striations can be seen run-
Ch. 8: Corundum Page of 311 Ch. 8: Corundum
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