24 
CORUNDUM, ITS orOURRENCE AND DISTRIBUTION. 
HARDNESS OF GEMS. 
The next table shows the relative position of corundinn with 
respect to hardness: 
Gem. 
Diamond 
Sapphire 
Ruby 
Spinel ruby . . _ 
Topaz 
Emerald beryl 
Beryl... ^ ... 
Hardness. 
10 
9 
8.8- 
9 
8 
8 
7.5- 
■ 8 
7.5- 
8 
Q-em. 
Zircon 
Rhodolite 
Pyrope 
Almandite 
Amethyst 
Hiddenite 
Hardness. 
7.5 
7.5 
7.5 
7.5 
7 
6. 5-7 
As is show^n by the above tables, sapphire is fifth in refractive index 
and second in hardness. Ruby is sixth and third, respectively; but, 
as is Avell known, it is not its brilliancy (which is dependent on index 
of refraction), but its rich pigeon-blood color, luster, and trans- 
parency which are so highly prized. It is these qualities, together 
with its hardness and rarity, that make the pigeon-blood ruby, when 
of more than one carat and free from flaws, the most valual)le of 
gems. 
The gems occur in the mines in three forms: F.irst, as crystals, of 
which there are two distinct types — the hexagonal prisms terminated 
by rhombohedrons and pyramids, sometimes with basal plane, the 
larger crystals being often rounded or barrel-shaped, and the flat, 
tabular crystals in which the basal plane is very largely developed: 
second, as transparent colored portions of larger massive pieces of 
corundum ; and third, as nodules of finer and clearer material in a 
mass of corundum in which parting has been highly developed. The 
nodules, when separated from the mass of corundum, often have the 
appearance of rolled pebbles. 
CORUNDUM. 
The varieties that are brought under this head are, with the excep- 
tion of emery, all those that can not be used as gems. As a com- 
mercial product there are differences, such as texture, purity, etc., 
that have considerable influence upon its value, in the same way in 
which color and transparency affect the gem corundum. Although 
the hardness of the pure corundum is practically the same — that is, 
9 — the cutting qualities of corundum vary, as has already been stated, 
according to the alteration that lias taken place in the mineral and 
to the development of parting planes. The usual colors of this 
