22 OORUNDTTM, ITS OCCURRENCE AND DISTRIBUTION. 
DESCRIPTION OF THE THREE VARIETIES. 
SAPPHIRE OR GEM CORUNDUM. 
ESSENTIAL PROPERTIES OF A GEM. 
As the terms " ^em mineral " and " preeioiis stone " are used at the 
present time tliey <ipply to three dasses of minerals: (1) Those which 
are of value for ciitting and use in jewelry and are commonly desig- 
nated as gems, as diamond, rnby, beryl, amethyst, etc.; (2) those 
which are used, after cutting or polishing, for ornamental or deco- 
rating purposes, as quartz, jade, agate, malachite, etc.; and (8) those 
which from their rarity and beauty are of ornamental or miner- 
alogical value, as quartz crystals, rhodochrosite, calcite, etc. As is 
evident, these divisions so overlap that no sharp lines can be draw^n 
between them. The word " gem " is used ordinarily to designate any 
precious stone wdien cut or polished. In mineralogy the term is often 
used for a class of minerals whose hardness is over 7 (or harder 
than quartz) and Avhich are without metallic luster and are brilliant 
and beautiful. The unit of weight of gems is designated by the 
term '* carat,'' and the international carat is equal to 8.168 grains or 
250 milligrams. This meaning of the Avord should not be confused 
with that which refers to the fineness of a gold alloy. 
The properties of a mineral that determine its value as a gem 
are its rarity, hardness, color, index of refraction, and luster. As an 
illustration of how the hardness affects the value of a mineral for 
gem purposes, sphalerite might l)e cited. This mineral, with an 
index of refraction and luster not far from that of the diamond, has 
a hardness of only 3.54, which effectually excludes it from being a 
gem mineral. 
SAPPHIRE AS A OEM. 
Corundum has properties that place some of its varieties among 
the most valuable gems. With the exception of the diamond, it is the 
hardest mineral known, and the rarity and color of the ruby, the red 
corundum, has made that gem, when more than a carat in weight, 
more valuable than a diamond of corresj^onding weight. Corundum 
has been found in almost all the colors of the rainbow, and in the fol- 
lowing list its gems have been classified according to color. They are 
very often designated by the prefix " oriental,'' to distinguish them 
from gems of the same name whose mineral composition and char- 
acter ar-e entirely different. 
