the Corundum Stone , and its Varieties , &c. 279 
made in a natural direction. Moreover, I have examined a great 
number of crystals of sapphire, many of which had one of 
their extremities, many others both their extremities, broken 
in a direction approaching more or less to that which is per- 
pendicular to their axes, but have never seen, among these 
fractures, any one that had the appearance of being made in 
the natural direction of the laminae; although, at the same 
time, I have, in many crystals, seen fractures which were per- 
fectly even, and often of considerable extent, in the direction 
of the planes of a rhomboid, exactly similar (with respect to 
the measure of its angles) to that belonging to the primitive 
crystal of imperfect corundum. I have already observed that 
there sometimes remain, upon the terminal faces of the crystals 
of sapphire, small facets belonging to the above planes. 
I cannot help mentioning also, in this place, a very interesting 
crystal of sapphire, that is in Mr. G-reville's collection* This 
sapphire, which is of a pale blue colour, is a simple hexaedral 
pyramid, the solid angle of whose summit measures 40°, and re- 
tains upon one of the angles of the summit, which is incom- 
plete, a large triangular facet, belonging to one of the planes of 
the primitive rhomboid. This plane is striated transversely, in 
a manner that shews some derangement in the crystallization, 
perhaps from too great rapidity; and, in the upper part, a still 
more rapid decrease changes its degree of inclination, causing 
it to take one which is greater, and which belongs to the se- 
condary obtuse rhomboid already described, in speaking of the 
fifth modification. These planes, together, completely terminate 
. the crystal at this extremity, in the manner represented in Fig, 
gg. There may also be observed, two other planes, between 
which is comprehended the plane I have just described as one 
Oo 2 
