18 OORUNDMM, ITS OCCURRENCE AND DISTRIBUTION. 
fracture, so that only the higher luiinbers of the commercial corundum 
(coarser grades) show the traces of this parting developed in them. 
In some the parting is so developed that even in the finer numbers the 
particles of corundum still show parting, which destroys greatly 
their cutting quality or efficiency. In crushing the ores in which the 
parting is developed to this extent, a great deal of fine powder called 
ftour corundum is produced, thus causing a waste of considerable 
material. 
If this parting is developed in coriuidum which has been made 
into a wheel, the cutting poAver is greatly reduced, for the corundmn 
grains, instead of breaking with an irregular fracture and pro- 
ducing a cutting edge, split along the planes of parting, and thus 
tend to produce a smooth surface. 
FRACTURE. 
The fracture of corundum is irregular to conchoidal, and it is this 
kind of fracture that is necessary to give the best cutting i)ower to 
the grains. If, however, the parting planes exist, the corundum 
grains will break along them, since they offer the least resistance, and 
the value of the corundum is thereby materially decreased. 
HARDNESS. 
Next to the diamond, corundum is the hardest mineral known. 
The hardness varies, although but slightly, with the different varie- 
ties. The value usually ascribed to corundum is 9, which is that of 
the blue sapphire variety, but all corundums do not reach this value. 
The ruby is slightly less hard, ranging from 8.8 to about 9. 
In emery it is the corundum which causes the high degree of hard- 
ness, even though the grains of corundum themselves can not always 
be distinguished. 
ABRASIVE EFFICIENCY. 
The hardness of corundum must not be confused with its abrasive 
efficiency, for, although most corundums vary but slightly in hard- 
ness, there is often a wide variation in the amount of a]:)rasion that 
they are able to accomplish. The hardness represents the resistance 
offered by the corundum to abrasion or to being scratched by other 
substances and also its power of scratching other substances. Any 
fragment of corundum that is entirely free from alteration or 
decomposition will, when tested as to its hardness, be found to be 
close to 9, and will scratch any of the minerals, except the various 
varieties of corundum and the diamond. This same material, how- 
ever, may have a cutting efficiency that is very much below that of 
another corundum whose hardness is just the same. 
