the Corundum Stone , and its Varieties , &c, $ig 
octaedron, its primitive crystal, a tendency to the replacing of 
all those edges ; an appearance which is very common in the 
octaedron of the diamond. I shall remark also, that the surface 
of its crystals has generally less lustre than is commonly ob- 
served in the crystals of spinelle. 
The desire of contributing every thing in my power, to render 
as complete as possible our knowledge respecting this substance, 
which has been but lately known to mineralogists, induces me 
to add to the variety of forms that have been described by the 
Abbe Hauy, those represented in Figs. 53 and 54, although 
the Ceylanite to which those figures belong comes from a dif- 
ferent place. The first is nothing more than the modification 
represented by the Abb6 Hauy in Fig. 104, Plate L, of his 
work, but in which the four planes that have replaced each of 
the solid angles of the octaedron, are situated upon these same 
angles, in the primitive crystal itself, instead of being situated 
upon the planes that have replaced the edges. I have fre- 
quently seen these planes encroach upon each other, to such a 
degree as to render it very probable that there exists, in the 
Ceylanite, that form of crystal which consists of 24 trapezoidal 
facets, and which, by its derivation from the cube, the regular 
octaedron, and the regular dodecaedron, is already so very com- 
mon in crystallography. 
The second of the forms just spoken of, (Fig. 54.) is the 
same variety, but with a very slight replacement of the edges 
of the octaedron : it is the beginning of the change to the above- 
mentioned Fig. 104, of the Abb£ Hauy. These two varieties 
belong to the Ceylanite which is inclosed in pieces of stone 
brought from Somma; and are indeed the most common 
T t 2 
