brittle. Its hardness is marked, being excelled amongst minerals by the Diamond alone. It is the Standard 9 of Mohs' scale.
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.
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-