PHYSICAL PKOPKinJES. 19 
The abrasive efficiency of a iniiieral is (lepeiideiit on its liardness 
and its fracture. If the mineral is hard and the i>rains break and 
wear away in such a manner that instead of l)ec()uiin<j^ rounded <»:ood 
cutting edges are kept exposed, tlie abj-asive efliciency is liigh. 'Hiis 
efficiency is measured by the amount of abrasion u])on a given surface 
in a given time. 
Many experiments have been made, and in a number of ways, to 
measure the abrasive efficiency of a corundum; but it is becoming 
apparent that the only safe method to determine its cutting power 
is to have it made up into Avheels and to have these tested as if they 
were in actual use. 
SPECIFIC GRAVITY. 
The specihc gravity of corundum is about 4, varying from 3.95 to 
4.10. This high specific gravity is of considerable assistance in recog- 
nizing corundum in the field, for there are but few of the nonmetallic 
or light-colored minerals that have a sj)ecific gravity as high as this, 
and none of these are liable to be found in the corundum regions. 
OPTICAL PROPERTIES. 
The luster of corundum is adamantine to vitreous, while that of 
emery is metallic to submetallic. On the basal surface of corundum 
the luster is sometimes pearly. 
Pleochroism in ordinary light is very strongly marked in the deeply 
colored varieties, especially the sapphires and rubies, the ruby show- 
ing a deep red color when viewed in the direction of the vertical 
axis, and a nuich lighter color to nearly colorless in some instances 
when viewed at right angles to this axis. The sapphire exhil)its a 
deep blue color wlien viewed in the direction of the vertical axis, and a 
greenish to greenish Avhite or bluish white when viewed at right angles. 
By means of this pleochroism exhibited by corundmn, the stones are 
readily distinguished from spinel, garnet, and other gem minerals, 
which resemble some of the corundmn gems. 
The action of the Roentgen rays or X rays upon corundum gems is 
another means of distinguishing the ruby and the sapphire from other 
minerals which resemble them, and from artificial or imitation stones. 
-Corundum allows these rays to pass through it freely, being exceeded 
in this respect only by the diamond, which allows the passageof ten 
times as much light. According to their resistance to the i)assage of 
the X rays, Doctor Doelter " has arranged the minerals into the fol- 
the X rays. Doctor Doelter « has arranged the minerals into the follow- 
ing groups, the diamond allowing the most light to pass through it: 
« Separat Abdruck aus Mitthoil. naturwiss. Ver. f. Steiermark, .Tahrg. 1895, Mar. 26, 
1896 ; and Seventeenth Ann. Kept. U. S. Geol. Survey, pt. 3, 1895-96, p. 922. 
