the Corundum Stone, and its Varieties, &c. 317 
axis. In Sir John St. Aubyn’s collection, I have placed a de- 
tached crystal, which has also a dodecaedral prism ; one of the 
extremities of this crystal is of a green colour.* 
Lastly, I have, in this same sand, met with a crystal, per- 
fectly colourless, the prism of which is completely triedral ; 
* The scarcity of the red tourmalin of Siberia, which hitherto has been known only 
by very small specimens, for which the dealers demand an extraordinary price, seems 
to be what has hitherto prevented naturalists from forming a decided opinion respecting 
its proper place in the system of minerals. I am therefore happy in announcing, that 
there is in Mr. Greville’s collection, a specimen of this kind of tourmalin, (from 
India,) the size and perfection of which are truly admirable. This specimen, which is 
not accompanied with any kind of matrix, is nearly as large as a man’s head ; and is 
entirely composed of crystals placed by the side of each other, in a diverging form, or 
rather penetrating each other at one of their extremities, and separating or diverging 
a little at the other extremity. Every one of these crystals, most of which are as long 
as the height of the specimen, is nearly as thick as the little finger. Their form is a 
hexaedral prism, which is deeply striated, and terminated by a triedral pyramid with 
rhombic planes, the angles of which, measure exactly the same as those of the corres- 
ponding pyramid in the common tourmalin. All the crystals are pretty transparent j 
and terminate on the top of the specimen, by the forementioned pyramids, but at dif- 
ferent heights ; a circumstance that gives to the top also a triedral pyramidal form, 
but much less obtuse than that belonging to each crystal of which it is composed. 
The greatest part of this specimen is of a pale purplish red, or flesh colour; but, to- 
wards the base, this colour grows much more deep, so that, at last, it becomes abso- 
lutely black. I have observed the same division of colour, in specimens of this red 
tourmalin from Siberia. 
The superb specimen here described was brought from the kingdom of Ava: it was 
given by the sovereign of that country, as a present of very great value, to Colonel 
Symes, who was sent on an embassy to him, by the English government. Colonel 
Symes placed it in Mr. Greville’s collection; and he could not possibly make a 
better use of it ; that collection being, in my opinion, one of the finest in Europe, 
with respect to the beauty of the specimens and the instructive series of each sub- 
stance which composes it, and certainly superior to all others, with respect to precious 
Stones in a state of perfect crystallization. 
The Abbe Hauy, in his Mineralogy, expresses a wish, that the prismatic enneae- 
dral form, terminated by the triedral pyramid of the primitive rhomboid, (which he 
MDCCCII. T t 
