THE COLOR OF AMETPIYST, ROSE, AND BLUE VARIETIES 



OF QUARTZ. 



By TiioMAS L. Watson and R. E. Beauu, 



Of the University of Virginia. 



INTRODUCTION. 



There is a surprising lack of exact information on the cause of 

 color in minerals, even in many of the common varieties. This is 

 especially true of minerals possessing exotic color in contradistinc- 

 tion to those having natural color, dependent on chemical com- 

 position. The literature is largely a foreign one, is comparatively 

 small but scattered, and much of the earlier work is of doubtful 

 value. Again, there is an almost entire lack of reliable analyses 

 involving accurate determinations of the minute quantities of 

 colorativc substances (pigments) diffused through minerals v/hich 

 exhibit exotic color. 



Decided progress, however, has been made in recent years in the 

 study of cause of color in dispersely colored minerals, many of 

 which are known to be very sensitive to high temperatures and 

 in some cases even to daylight. The chemical nature of the coloring 

 substances is less well known and opinion on the subject is divided. 

 Heating tests for destroying color and its restoration on exposure 

 to the emanations of Roentgen, cathode, or radium rays are used to 

 distinguish between colorative substances of organic and inorganic 

 nature. Also by the use of the ultramicroscope and tiie apphcation 

 of the principles of colloid chemistry the nature and composition of 

 the substances causing color may be determined. 



The present paper gives the results of some experiments recently 

 completed by the writers on the color of several varieties of quartz, 

 and a brief summary and discussion of the work by others on the 

 same varieties of the mineral.' There is included on page 559 a note 

 by George Steiger, Chemist to the United States Geological Survey, 

 of some quantitative determinations made of manganous oxide 

 (MnO) in rose quartz from Creede, Colorado, and in amethyst from 

 an unknown locality. Especial attention is directed to the analyses 

 of amethyst and rose quartz from many difYcrent locahties tabulated 

 on page 554. 



• The HTiters are Indebted to Dr. Edgar T. Wherry, of the United States National Museum, for bolpfUl 

 suggestions offered on reading the manuscript. 



Proceedings U. S. National Museum. Vol. 53-No. 2220. 



