6 BULLETIN 118, UNITED STATES NATIONAL MUSEUM. 



minerals, and render them yellow, brown, or red. 2, Disperse colors, 

 due to submicroscopic inclusions, and varying with their shape, size, 

 or arrangement. These colors are thought to have been developed, 

 in part at least, by exposure in the earth to radio-active substances, 

 slight decomposition having thereby been effected, and traces of the 

 constituent elements set free. Heating usually destro} r s these colors, 

 since it causes the free elements to reunite, but exposure to radiant 

 energy of various kinds often restores them. 3, Internal reflection 

 colors, due to symmetrically arranged inclusions, or to lamellae of 

 extreme thinness, producing interference of light. 



A special color phenomenon remains to be considered. Some 

 minerals transmit light of different colors in different directions, and 

 are said to be pleochroic. The most striking instance of this among 

 precious stones is iolite, which is sometimes called "dichroite" be- 

 cause of this very property; in one direction it appears intense blue, 

 in another somewhat paler blue, and in the third pale yellow. In 

 many cases, however, the difference in color is less striking, and special 

 means must be used in order to detect it. An instrument, called a 

 dichroscope, is sometimes used for this purpose. It consists of a 

 metal tube containing a cleavage piece of Iceland spar; at one end it 

 is pierced with a small square hole, and at the other has a magnify- 

 ing lens to serve as an eyepiece; the hole appears double when viewed 

 through the latter. When a pleochroic stone is placed in front of 

 the square hole, the two images of the hole will be differently colored. 



Another method of observing pleochroism makes use of the polariz- 

 ing nicol prism in the microscope. The stone is placed on the stage, 

 and the polarizer introduced; the stone is observed in one position and 

 again after turning the stage through 90°; one of the pleochroic 

 colors will be seen in each of these positions. 



It should be noted that isometric minerals, which are isotropic 

 between crossed nicols, show no pleochroism; tetragonal, hexagonal, 

 and trigonal ones may show two colors, while those of the three re- 

 maining crystal systems in general show three colors. 



Luster. — The luster or brilliancy of a mineral is a physical pheno- 

 menon connected with the manner in which light is refracted by it; 

 the principal types are metallic, adamantine, and vitreous, but in 

 addition some minerals of fundamentally vitreous luster possess cer- 

 tain peculiarities of structure, and as a result exhibit lusters resem- 

 bling those of familiar substances, as waxy, greasy, silky, etc. 



Hardness. — On the hardness of a mineral is dependent its resistance 

 to scratching or abrasion. It is usually described by reference to a 

 standard scale of 10 minerals, which are, beginning with the softest — 

 1, talc; 2, gypsum; 3, calcite; 4, fluorite; 5, apatite; 6, orthoclase 

 or microcline; 7, quartz; 8, topaz or beryl; 9, corundum (ruby or 

 sapphire) ; and 10, diamond. Each of these will scratch all pre- 



