658 rROCEEDINGS OF THE NATIONAL MVSEU2I. vol.53. 



In 1904 Wherry^ suggested that the variety of amethj^st quartz 

 found in Delaware County, Pennsylvania, was probably colored by 

 a titanium compound, since the microscope showed the presence of 

 rutile inclusions. This might well be questioned in light of the 

 investigations by the senior writer and others on blue quartz, which 

 is discussed below. 



In 1906 Berthelot^ showed that when amethyst is decolorized by 

 heating it regains its violet color on exposure to radium rays. Ho 

 found that quartz and glass containing small amounts of manganese 

 behaved similarly; hence the color of amethyst was referred by him 

 to a slight manganese content. 



The investigation by Simon^ in 1908 represents the most important 

 contribution thus far made on the color of minerals. By destroying 

 the color of a number of dispersely colored minerals in oxidizing 

 (oxygen), reducing (hydrogen), and when necessary neutral gases^ 

 and examination of the chemical character of the resulting gases or 

 distillates, Simon concluded that the colors of the five minerals 

 investigated by him (zircon, amethyst, smoky quartz, tourmaline, 

 and Brazilian topaz) wei^e produced by unstable inorganic com- 

 pounds. He states that these unstable inorganic compounds are 

 simply rearranged at elevated temperatures but not destroyed, 

 since exposure to radium rays or even to sunlight will restore them to 

 their original condition. According to Simon, the exact nature and 

 constitution of the coloring substances are not laiown. 



The work of the present writers eliminates, it is believed, the 

 probability of inclusions of foreign substances of visible microscopic 

 size as a cause of amethystine color in quartz. While minute inclu- 

 sions of varying solid and liquid substances were present in the 

 thin sections of amethyst examined microscopically, they were of 

 such character as regards distribution and quantity as not to be 

 seriously considered as a cause of color. In the absence of ultra- 

 microscopic study, the existence of foreign substances of submicro- 

 scopic size can neither be denied nor affirmed. 



The analyses made of amethyst from different localities and 

 tabulated on page 554 are important in showing appreciable quantities 

 of the iron, manganese, and titanium oxides — mctalhc oxides wliich 

 are known to affect the color of minerals. Also the cobalt oxide, a 

 highly colorative substance even in small quantity, was found in 

 each of the four analyses of amethyst in quantity sufficient to impart 

 a distinct blue color when taken up in a borax bead. Examination 

 of the analytical results discloses the fact that both iron and titanium 

 oxides exceed in amount several times that of manganese oxide. 



» The Mip.p.ral Collector, 191)J, vol. 11, p. 88. 

 » Compt. Hend., 1906, vol. U3, p. 477. 



» Neues Jahrb. f. Min. etc., 1903, vol. 26, pp. 249-295. Translated by Edgar T, Whsrry, Mineral Collector, 

 1908-9, vol. 15, pp. I60-X68, 177-182. 



