OIST THE RUBIES OF BURMA AND ASSOCIATED MINERALS. 
221 
In a recent communication to the Mineralogical Society* 1 have shown that in the 
corundum, which has not been subjected to pressure and thus had gliding planes 
developed in it, the fracture is conchoidal like that of quartz. In such untwinned 
Fig. 18. 
crystals there is a plane of chemical weakness parallel to the basal plane (OR., 100). 
This is shown by the frequency of a pearly lustre on that face, due to the development 
of films of diaspore within the crystal, and sometimes also by a tendency of the crystal 
to break up parallel to this plane. Now the presence of this plane of chemical weak¬ 
ness is very strikingly exhibited by some Burmese rubies. 
Fig. 18 represents a much-altered ruby crystal, magnified four diameters. The 
basal plane has been attacked irregularly, and the deep holes u and b show how 
capriciously such erosive action sometimes goes on ; at c we have a rhombohedral face 
undergoing the kind of exfoliation, of which we are about to speak as characteristic 
of those planes of the crystal, but at its upper part, d, we see that the crystal is made 
up near the much-weathered basal plane of alternate layers of corundum and diaspore. 
The same fact is still better shown on the fractured surface f It is this alternation 
of diaspore layers which gives the pearly lustre so often exhibited on the basal plane 
of corundum crystals. 
When, in consequence of pressure, gliding planes (similar to those produced in 
calcite) have been formed in corundum, chemical action tends to take place along 
these gliding or secondary twinning planes. 
In rubies embedded in the limestone, the faces parallel to the rhombohedron are 
* ‘Mineralogical Magazine,’ vol. 11 (1895), p. 49. 
