CORTTNDUM. 25 
ordinaiy corundum are gray to white, shades of bhie, white mottled 
with bhie, and also the darker colors, broAvn to black. 
According to its structure, corundum is divided into three groups, 
known as (1) block corundum, (2) crvstal corundum, and (8) sand 
corundum. These three varieties of corundum, which are present 
at the different mines, are sometimes all three found in the same mine, 
although not all three in the same vein. 
BLOCK CORITNDITM. 
Block corundum includes the massive corundum, whether in small 
or large masses. A maSvS of block corundum weighing over 5,000 
pounds has been reported to have been taken from the Laurel Creek 
mine, Rabun County, (la. This mine has probably furnished the 
largest blocks of corundum of any in the world. In some of the 
deposits the block corundum is often intermixed with feldspar, horn- 
blende, nuiscovite, margarite, or chlorite, etc., according to the char- 
acter of the rock in which it occurs, so that the separation of the 
corundum from these foreign minerals is sometimes a rather difficult 
process. Whei'e the corundum occurs in masses of considerable 
weight, there is often great inconvenience in mining, as, on account 
of its toughness and hardness, it is not always readily l)roken and it 
is almost impossible to drill through it. The block corundum, which 
shows but little development of the parting planes already referred 
to and no ingrowth of muscovite, margarite, or chlorite in cracks 
or seams, makes the l)est corundum ore, and the difficulty of cleaning 
is reduced to a minimum. 
CRYSTAL f ORITNDUM. 
Under this head are included all the crystal varieties of corundum. 
These are j^resent in deposits of both sand and block corundum. At 
many of the localities the crystals show the hexagonal prism merg- 
ing into the jiyramid, thus causing the crystal, as it tapers toward 
the end, to assume the form known as " barrel corundum.'' At a 
number of mines loose, tapering crystals of rather indefinite form 
are found, which are inclosed by compact margarite. At many of 
the veins the crystals occur in a mass of feldspar, at others in 
biotite or nuiscovite, and at still others in chlorite. 
SAND CORUNDUM. 
Sand corundum consists of very small to minute crystals and small 
irregular grains, such as are found in the chlorites and vermiculites 
developed in the ore bodies occurring between the peridotite and other 
rocks, such as gneisses and schists. 
