Mr. Greville on the 
An Analytical Description of the Crystalline Forms of Corundum, 
from the East Indies , and from China. By the Count de Bournon. 
The most usual form of Corundum is a regular hexaedral 
prism ; (Tab. XXII. fig. 1.) in general, the surface of the crys- 
tal is rough, with little lustre, owing to unfavourable circum- 
stances under which it crystallized. 
The crystals of Corundum hitherto found were not formed in 
cavities, where, each crystal being insulated, its surface could 
preserve that smoothness and natural brilliancy which are com- 
mon to all substances that freely assume a crystalline form. 
Like the crystals of feldspar which we meet with in the porphy- 
roid granites, the Corundum crystals have been enveloped, at 
the time of their crystallization, by the substance of the rock 
which was forming, at the same time with themselves, in an 
imperfect and confused crystalline mass ; and the Corundum 
crystal, before it had acquired its perfect solidity, necessarily 
received on its surface the impression of the different particles 
of the rock which enveloped them : this naturally renders the 
surface rough and dull. Crystals of feldspar found in the gra- 
nitic porphyroid rocks, exhibit the same kind of appearance,,, 
from the same cause. 
The Corundum crystals are in general opaque, or at least 
they have only an imperfect transparency at the edges : when 
broken into thin fragments, the pieces are semi-transparent : 
when held between the eye and the light, and examined with 
a powerful lens, it will be perceived that their interior texture 
is rendered dull by an infinite number of small flaws crossing 
