500 REPORT OF NATIONAL MUSP^UM, liXX). 



eye." C'rocidolito contiiins in 100 parts: Silica, 51; ii-on oxides, 84; 

 soda, 7; iiiao-nosia, 2; Avater, 3. 



The ])est specimens occur in Gri(iualand and the Oranj^e River 

 country, South Africa. These are essentially (juartz, pseudoniorphous 

 after crocidolite, and have a hardness of about 7 and a specific gravity 

 of 3.2. (Plate 2.) 



DAMOURITE. 



This mineral is one of the micas and usuallj'^ results from the altera- 

 tion of some other mineral. It is of little use as a gem. Practical Ij'^, 

 the oidy locality at which the mineral is so used is Stoncham, Maine, 

 where a green and red damourite, altered from topaz, has been cut 

 into trinkets. 



DATOLITE. 



Datolite is another of the mineralogical gems. Its hardness is 5; 

 specific gravity, 2.98; color, white, creamy, grajnsh, pale green, yel- 

 lowish, reddish, or amethystine. It occurs in small, glass}^ crystals 

 and massive, often having a i-adiating structure. 



One hundred parts contain: Silica, 37.7; boric acid, 21.8; lime, 

 31.0; water, 5.6. 



DIAMOND. 



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 

 propert}^ of dispersing light — that is, dividing light into colored raA's, 

 causing that peculiar flash of prismatic hues called its jire. The dia- 

 mond crystallizes in the isometric system, usually in octahedrons, or 

 combinations of octahedron, cube, dodecahedron, and tetrahedron, the 

 crystals having their faces commonly curved. The cleavage of the 

 diamond is highly perfect and parallel to the octahedral faces. The 

 luster, especially of artificial faces, is peculiarl}' ])rilliant and is .superior 

 to that of any other gem. The remarkable brillianc}^ of the diamond 

 residts 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 21 

 degrees. The stone also refracts light strongh'. To the refractive 

 and dispersive power of the diamond are 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, att'ord 

 the greatest number of shades and arc 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, how- 

 ever, very rare, as, indeed, are all the strongly colored specimens. 

 Red stones of strong, rich, deep tints ai"e extremely j-are; so, too, are 

 the garnet, hyacinth, rose-red, peach-blossom, and lilac colored speci- 



