the Corundum Stone , and its Varieties , &c. 251 
Whatever the form of the crystals of this substance are, they 
may always, by dividing them, be ultimately brought to the 
rhomboid here spoken of ; and, when they are broken, such of 
the fragments as are made in the direction of the laminae, very 
often present the same rhomboid, in a very regular form. In- 
deed, it is the only method of obtaining this crystal, in the 
' imperfect corundum ; for, among all the crystals of that kind 
of corundum which have been sent from the East Indies, not one 
has yet presented its primitive form. With respect to the per- 
fect corundum, I have been more fortunate ; as, besides several 
fragments which exhibited this rhomboid very exactly, I have 
found four of these primitive crystals perfectly defined. One of 
them is a sapphire, and is in the collection of Sir John St. 
Aijbyn ; the three others are oriental rubies, and are in the col- 
lection of Mr. Greville. 
First Modification, The summit of the pyramid, (as very 
frequently happens in calcareous spar, and in most of the 
stones which have a rhomboid for their primitive form,) is often 
replaced by a plane which is perpendicular to the axis. This 
plane then makes, with those of the rhomboid, an angle which 
differs very little from 122 0 go'; and, as the extent of the plane 
is more or less considerable, it often causes great difference 
in the appearance of the crystals. Sometimes it does not descend 
so low upon the faces of the rhomboid, as to reach their small 
diagonal. (Fig. 2.) At other times, it exactly reaches to the 
diagonal. (Fig. 3.) And, very often, it descends more or less 
below it. (Fig. 4). This last variety is frequently met with in the 
perfect red corundum, or oriental ruby. 1 also know four in- 
stances of this form in the sapphire. The variety shown in Fig. 3 
is rather scarce ; but that of Fig. 2 is the most rare of the 
