274 MINERALOGY 



reflected as the color of the surface, while the depth or tone is 

 modified by the physical condition of the surface, for a highly pol- 

 ished surface will glisten with the quantity of light reflected, and a 

 rough, earthy, or powdery surface will appear dull from the amount 

 of light diffused. Combined with these surface effects, in trans- 

 parent minerals, is the color of transmitted light, characteristic 

 in some cases, as in crocoite, of the compound, in others, as in 

 quartz, due entirely to impurfties. 



Since the nature of the surface is variable, it has been found more 

 accurate in comparing the color of mineral specimens to use the 

 very finely ground powder, which insures a surface always of the 

 same nature, from which the same amount of light will be reflected 

 or diffused, and the color will not depend upon fortuitous causes. 

 The most convenient method of obtaining the fine powder quickly 

 is to draw, with a firm pressure, the rounded corner of the 

 specimen quickly across an unglazed porcelain surface, when 

 most minerals will leave a mark, termed the streak, the color 

 of which is that of the fine powder. The color of the streak is 

 of great help in the rapid determination of minerals. The streak 

 is little affected by the structure of the specimen, as hematite 

 occurs in well-formed crystals, micaceous, compact, massive, and 

 earthy, with a blood-red color, gray, steel-gray to nearly black, 

 iridescent, or brown; all of these varieties will yield a cherry-red 

 streak. 



A mineral is described as of metallic luster when the streak is 

 dark in color and the specimen is opaque on the thin edges or in thin 

 sections and has a shiny surface, common to most metals, as tin 

 or iron. When the streak is dark and the surface is not shiny, it is 

 of submetallic luster. 



If the streak is light in color or the thin edges and sections trans- 

 mit light, it is non-metallic in luster. 



The color of metallic and submetallic minerals is more character- 

 istic and less variable than in non-metallic. It is therefore more 

 reliable as a means of identification. 



The true color of a mineral is shown only on a freshly broken 

 surface, as by oxidation or weathering the natural surface may be 

 entirely changed. The beautiful iridescence of some pyrites, 

 chalcopyrite, limonite, and hematite, is caused by a thin film of 

 oxides or hydroxides which yield spectral colors through the inter- 

 ference of light. 



The surface change in color due to chemical change is termed 



