CATALOGUE OF GEMS AND PRECIOUS STONES. 45 



The diamond is the hardest of gems; is the only one that is com- 

 bustible; is the most highly refractive; and surpasses all others in the 

 property of dispersing light — that is, dividing light into colored rays, 

 causing that peculiar flash of prismatic hues called its fire. The cleav- 

 age of the diamond is highly perfect and parallel to the octahedral 

 faces. The luster, especially of artificial faces, is peculiarly brilliant 

 and is superior to that of any other gem. The remarkable brilliancy 

 of the diamond results in part from the total reflection of light from 

 its internal faces when the incident ray strikes it at an angle of a little 

 more than 24 degrees. The stone also refracts light strongly. To 

 the refractive and dispersive power of the diamond is due the flash 

 of colors or fire, characteristic of the stone, the colorless specimens 

 exhibiting it to the greatest degree; the colored to the least, or not 

 at all. 



The range of color of the diamond is extensive, including nearly 

 all the prismatic hues. The whites, yellows, and browns perhaps 

 afford the greatest number of shades and are the most numerous. 

 Next to these, for colored specimens, the greens, including all shades, 

 are most plentiful; the pure grass-green and emerald-colored stones 

 are, however, very rare, as, indeed, are all the strongly colored speci- 

 mens. Red stones of strong, rich, deep tints are extremely rare; so, 

 too, are the garnet, hyacinth, rose-red, peach-blossom, and lilac- 

 colored specimens. Practically all of the blue stones known have 

 been found in India, and their occurrence is as rare as the red. Cin- 

 namon, brown, black, milky, and opalescent stones are occasionally 

 met with. Pure colorless diamonds without a flaw or tint of any 

 kind are more rare than is generally supposed. 



In diamonds, perfectly white stones or decided tints of red, rose 

 green, or blue are most highly prized. Fine cinnamon, salmon, 

 brown, black, or yellow stones are also much esteemed. When flawless 

 and without tint of any kind they are termed "first water. " If they 

 possess a steely-blue color they are called " blue white. " It is impos- 

 sible to estimate the value of a diamond by its weight. Color, bril- 

 liancy, cut, and general perfection of the stone are all to be consid- 

 ered. Of two stones, both flawless and of equal weight, one may be 

 worth twice the other. Off-colored or defective stones may sell at 

 carat prices regardless of size, while the value of an ordinary good 

 water stone increases in an increasing ratio with its weight up to about 

 20 carats, beyond which no rule holds good. Exceptionally perfect 

 stones have no fixed value, the price depending upon the purity and 

 brilliancy and, of course, the condition of the market. 



Among the historically interesting diamonds are the "Regent" 



or "Pitt" diamond, weighing 136f carats, for many years the finest 



large diamond in the world. It was discovered in India in 1701, and 



weighed 410 carats in the rough. The finest blue diamond is the 



4555—22 5 



