10 CORUNDUM TN THE UNITED STATES. [bull. 180. 
sapphire as the oriental ruby, the green sapphire as the oriental 
emerald, the yellow sapphire as the oriental topaz, and the purple 
sapphire as the oriental amethyst. There are also pink and white 
sapphires. The corundum gem or sapphire localities are usually dis- 
tinct from the corundum localities, although very handsome gems 
have been found where the corundum was mined for abrasive pur- 
poses, notably at the Corundum Hill mine, Cullasagee, N. C. 
Corundum as it is mined for abrasive purposes occurs as (1) sand, 
(2) crystal, or (3) gravel and block corundum. Sometimes all three 
types are found in the same deposit. The sand corundum consists of 
small grains or fragments of the mineral scattered through the vein. 
The crystal corundum consists of crystals up to 3 inches in length. 
Often these crystals have parting planes so thoroughly developed that, 
in crushing, the corundum breaks up into regular rhombohedrons, 
this breaking continuing to even the finer sizes and causing the grains 
of corundum to crumble when in use. This persistent regular break- 
ing of the corundum destroys its cutting efficiency, for it is the irregu- 
lar fracture that produces the best cutting edge. The block variety 
often occurs in masses of almost pure corundum from 10 to 1,000 
pounds in weight. Again it occurs in large masses intimately asso- 
ciated with hornblende, feldspar, etc., making a rock which is tough 
and is difficult to work. Frequently the only way to break the masses 
is to build fires over them and then suddenly cool them by pouring- 
water upon them. The parting x^lanes are at times very noticeable in 
the block corundum, and are, as in the crystal corundum, detrimental 
to the commercial product. The parting planes are not lines of cleav- 
age, but are planes developed in the mineral by synthetic twinning, 
usually parallel to the unit rhombohedron. 
It is the hardness of corundum that makes it of so great value as 
an abrasive. Next to the diamond it is the hardest mineral known, 
having a hardness of 9, while the diamond has a hardness of 10. 
Garnet, which is sometimes used as an abrasive, has a hardness of 
but 7 to 7.5. Corundum varies slightly in hardness; the sapphire 
varieties are generally considered the hardest, and of these the blue 
stands at the head. Some corundum has been observed that was 
readily scratched with a knife, as that from Acworth, Ga. The hard- 
ness of a corundum is often misjudged because, when made into a 
wheel, it does not cut so well as another, when in reality the degree 
of hardness may be the same in both cases, but the cutting efficiency 
of one surpasses that of the other. 
The theoretical composition of corundum is alumina (A1 2 3 ); but 
with one or two exceptions all corundums that have been examined 
vary from this by containing a small percentage of other constituents, 
principally silica (Si0 2 ), water (H 2 0), and ferric oxide (Fe 2 3 ). Nearly 
all corundums contain water, the amount varying from a trace to 2 
per cent or more. The amount of silica and ferric oxide varies also, 
some corundums containing neither of these constituents and some 
