THE MICAS, ASBESTOS, AND GEMS 165 
Bort is a black variety of diamond, which grows in radial 
groups. It is used for cutting brilliants and for the dies 
in wire drawing. Carbonado is a massive black diamond, 
which has no cleavage, and is therefore tough as well as 
hard; it is found in pieces up to 3000 carats in weight ; 
it was used for the cutting rims of core drills until the rise in 
price led to its general replacement by cutting bits of iron or 
steel, and bv chilled shot. 
Tae CorunpuM GROUP 
The gems of the corundum group consist of oxide of 
aluminium (AL,Og). Their value depends on their colour 
as they lack the brilliance of the diamond. They crystallize 
in the hexagonal system, their hardness is number 0 on the 
scale, and they have no cleavage. Corundum crystallizes 
from a magma that contains an excess of alumina as quartz 
does from an excess of silica. Corundum is formed mostly 
with basic calcic rocks because in those rich in alkalis most 
of the alumina is used as felspar; spinel is formed in those 
rich in magnesia, and the alumina left is available for cor- 
undum. Morozewicz (Tsck. Min. and Pet. Mitt., xviii, 1899, 
Pp. 100-1, 240) showed that a nepheline-basalt when fused 
with glass dissolves alumina and throws it out during cooling. 
If the magma cool rapidly, as in dykes, the corundum may be 
distributed throughout the rock, but in slowly cooling masses 
it forms on the margin. Hence corundum in commercial 
quantities forms where an ultra-basic rock is intrusive into 
rock rich in alumina. Thus the corundum mines of North 
Carolina occur where dunite (Pratt and Lewis, N. Carol. 
G.S., i, 1905) intruded gneiss, and the dissolved alumina 
crystallized on the margin in an irregular sheet or pockets of 
corundum. 
Sapphire, the blue variety, has been formed in scattered 
crystals beside narrow intrusions of basalt in New South 
Wales (Curran, %.R. Soc. N.S.W., xxx, 1806, p. 235), in 
Mull (Mem. G.S. Scotl., Mull, 1924, p. 274), and Montana 
(Pirsson, Amer. ¥. Sci., (4); iv, 1807, p. 42). Most sapphires 
come from alluvial deposits in Ceylon and are doubtless 
derived from deep-seated contacts. 
The most valuable variety is the ruby, which is red and