%66 Count de Bournon's Description of 
might be broken, with almost as much ease, in a direction 
contrary to that of the laminae, as in the direction of the laminae ; 
but it most frequently happens, in this case, that the fracture, 
although made in the natural direction, has not the evenness 
such fractures usually have, but presents some irregularities, and 
likewise some conchoidal parts : this remark, however, applies 
only to such pieces as approach nearly to perfection, with respect 
to transparency. 
There may frequently be observed, in these stones, a cha- 
racter which serves to confirm what I have said respecting 
the imperfection sometimes observed in their crystallization, 
which appears to me to arise principally from a want of abso- 
lute contact between all the parts of their crystalline lamina. 
When some of the faces of the crystals correspond to those 
of the primitive rhomboid, whether these faces are natural ones 
or are produced by fracture, the edges of the crystalline laminae 
are shown upon them, and sometimes very plainly, by lines 
which cross each other, in such a manner as to form rhombs of 
g 6 ° and 84°. This character even becomes of great use in this 
substance, as it serves to distinguish, in fragments, (which are 
generally of hexaedral prisms, that being the most common 
form,) those faces which are occasioned by fracture, from those 
which correspond to the terminal faces of the prism. These last, 
also, frequently exhibit lines, which are likewise caused by the 
edges of the crystalline laminae ; but, as they extend to three 
only of the alternate angles of the terminal hexagonal face, they 
trace on it, by crossing each other, either equilateral triangles, 
or rhombs of 6 o° and 120°. Figs. 34, A, and 34, B, represent 
these two different appearances ; the first upon the planes of the 
rhomboid ; the second upon the terminal faces. 
