the Corundum Slone, and its Varieties, &c. 255 
considerable size, more or less evident traces of the planes of the 
primitive rhomboid; as appears by small isosceles triangular 
planes, of greater or less extent, situated upon three of the al- 
ternate solid angles, formed by the meeting of the terminal faces 
with those of the pyramid. (Fig. 16.) 
It very often happens, in this modification, that the plane 
which has replaced the solid angle of the summit, acquires a 
more considerable increase in one of the pyramids than in the 
other ; and indeed, most commonly, this increase is such as to 
cause the pyramid entirely to disappear. The crystal then be- 
comes a simple hexaedral pyramid, which is either complete, as* 
in Fig. 17, Plate VII. (but this very rarely happens,) or has its 
summit more or less replaced. (Fig. 18, A.) This variety, which 
is very common in the crystals of perfect corundum, is also fre- 
quently met with in those of the imperfect corundum from 
China ; and it is very usual to see, upon the solid angles of its 
terminal faces, small isosceles triangles, which are occasioned 
by the preservation of some parts of the planes of the primitive 
rhomboid; (Fig. 19.) but they are seldom so regular in their 
form as they are represented in the figure. 
I have often seen small crystals of oriental ruby that exhi- 
bited a very pretty variety, as they showed, at the same time, the 
primitive rhomboid with its summit strongly replaced, and the 
incipient change to the form of the hexaedral pyramid which 
constitutes this third modification : this variety is represented in 
Fig. 20. There are, in Mr. Greville's collection, two very 
perfect crystals of this form. 
The second modification, that in which the pyramids of the 
primitive rhomboid are separated by an intermediate hexaedral 
prism, is often combined with the abovementioned union of 
L 1 .2 
