25S Count de Bournon’s Description of 
the first and third modifications. There exists, for example, in 
Mr. Greville’s collection, an oriental ruby, which exhibits the 
variety shown in Fig. 20, with the rudiments of an interme- 
diate prism, as is seen in Fig. si. This variety is also sometimes 
found among the small transparent crystals of imperfect corun- 
dum from China. 
In four other crystals, also in Mr. Greville’s collection, the 
prism is very much elongated ; and the plane which has replaced 
the solid angle of the summit of the pyramid is much more 
extended, as in Fig. 22. These crystals, which are oriental 
rubies, are in perfect preservation at their two extremities. 
There is besides, in the same collection, another crystal, also 
an oriental ruby, which differs from the preceding, in having no 
traces left of the planes of the primitive rhomboid. The crystal, 
consequently, appears to be a regular hexaedral prism, with the 
edges of its terminal faces bevelled. (Fig. 23.) 
In fite others, the pyramid has made more progress ; and, in 
all of them are to be seen, on their terminal faces, some slight 
traces of the primitive rhomboid. (Fig. 24.) 
Lastly, in one other specimen, the pyramid is nearly complete, 
as in Fig. 25. I also know two sapphires, which exhibit an inter- 
mediate variety, between the two last-mentioned forms. 
One of the most striking characters of corundum is, the great 
variety exhibited by this pyramidal modification, in the incli- 
nation of the faces of the pyramids to the axis of the crystal, 
and, consequently, in the more or less rapid decrease that has 
taken place in the crystalline laminae, at the plane angles 
situated on the common base of the two pyramids which com- 
pose the primitive rhomboid. Among the crystals of imperfect 
corundum, from the different districts in which this substance 
