COKUNDUM, ITS OCCURRENCE AND DISTRIBUTION. 
15 
PHXSIGAI. PROPERTIES OF CORUNDUM. 
CRYSTAL FORM. 
Corundum crystallizes in the rhombohedral division of the hex- 
agonal system. There is considerable variation in the form of 
crystals from different localities, but in their 
common development they usually show the 
forms of the prism and the pyramid of the 
second order, «(1120) and ?i(2243), in combi- 
nation Avith the base c(OOOl), and the rhombo- 
hedron y'(lOll). This unit or finidamental 
rhombohedron differs but little in angle from 
the cube. 
There are two common types of crystals. 
One is prismatic, in which the prism a{1120) is 
the prominent form and is sometimes termi- 
nated simply by the base 6'(0001), fig. 1, and 
again by the base and the rhombohedron 
r(lOll), or by the base, the rhombohedron, and 
the pyramid V^( 2243), fig. 2. The other is flat 
and tabular, in which the basal plane is the 
prominent form, the crystals being often a 
combination of the base and the rhombohe- 
dron, 7% fig. 2, PI. V, or of the base and the prism, fig. 16, Z?, or a com- 
bination of the two, fig. 1 of PL V. 
Pyramidal crystals m which the pyramid 
of the second order /^2243) is the prominent 
form (fig, 3) are common in but few locali- 
ties. The more complicated forms of crystals, 
as in fig. 4, are rarer, and it is the simple 
crystals that are the most often observed. 
When the crystals are small they are 
usually well developed, with smooth faces 
and sharp edges; the larger crystals are 
rough, striated, often rounded, and taper 
slightly toward the end, like a barrel, so that 
they are sometimes called "barrel corundum." 
The basal plane often shows characteristic 
striations which are parallel to the intersec- 
FiG. 2.— Corundum crystal. ^ious of the basc c and the three faces of the 
rhombohedron r, as shown in fig. 5 of PI. V. The lines are often very 
sharp and distinct, especially on the tabular crystals. In other cases 
the basal plane is marked by a series of concentric; hexagons whose 
sides are parallel to the intersection of the base c with the prism of the 
Fig. 1.— Corundum crys- 
tal, prism terminated 
by base, Cowee Valley, 
N. C. 
a 
