PHYSICAL PROPERTIES. 17 
show any cleavage, l)iit broke Avitli a conclioidal to spliiiiery fracture. 
IVof. J. W. Judd'^ has shown that unaltered corunchnn does not 
exhibit any trace of cleavage. 
There are three crystallographic i)]aiics along wliich parting has 
been observed : 
1. Parting has been observed parallel to the base c (0001). Judd ^ 
says: "That this basal plane is a solution plane of corundum is 
shown by the fact that the crystals from Burma and other localities 
which have undergone partial conversion into diaspore and hydrous 
silicates exhibit a remarkable step-like structure (resembling that of 
the ' Babel quartz ' on a very minute scale), showing that the chemical 
disintegration of the crystal has been governed by the existence of 
these solution planes. This fact is beautifully illustrated in the fine 
ruby presented to the British Museum by Mr. Ruskin, which exhibits 
the shaly structure due to corrosion and the natural etched figures in 
a very striking manner. I have no doubt that this fine ruby came 
from Burma, and many small specimens which I have examined from 
the same locality exhibit similar characters. As in the case of 
murchisonite and diallage, the multiplication of cavities along certain 
planes gives rise to a plane of weakness or a parting scale in the 
crystal along Avhich it readily divides." 
The basal parting plane often shows a pearly, or sometimes sub- 
metallic, luster, which is decidedly different from the vitreous to 
adamantine luster of the crystal faces. This basal parting, although 
perfect over a portion of the crystal, may be interrupted, and its con- 
tinuation will be step-wise along a nearly parallel plane. In general, 
crystals can not be broken into plates with parallel faces, as would be 
the case if the mineral possessed a cleavage. 
2. The parting planes ])arallel to the unit rhombohedron r (1011) 
are twinning planes, which, it is believed, have been caused by pres- 
sure. It has been observed that a crystal without this rhombohedral 
parting may be destitute of any lamellar structure ; or, as Tschermak 
has pointed out, it may be developed on Iavo of the rhombohedral 
planes and be almost or entirely lacking on the third. 
3. This parting plane, which is parallel to the prism a (1120), is 
seldom observed by the actual breaking of the corundum crystals, but 
can be seen in thin sections under the microscope. It is best seen in 
the corundum from the Chantabon Hills, in Siam. 
The parting planes developed in corundum have a marked effect 
upon its value as an abrasive. Many corundums which exhibit this 
parting in the larger fragments soon begin to break with irregular 
"Mineral. Mag., vol. 11, 1895, p. 50. 
'' Loc. cit. 
Bull. 269—00 M- 
