THE EARTH'S ROCK FOUNDATIONS 



49 



into needles or threads. Cleavage, then, is another physical 

 feature that aids in the determination of minerals (Fig. 27). 



Certain minerals break in a characteristic way other than 

 along cleavage planes. The mineral is then said to possess a 

 peculiar fracture. Thus flint breaks with a conchoidal fracture, 

 the surface of the break being either concave or convex like a 

 clam shell. 



FIG. 27. Feldspar, to show cleavage 



The fresh surface of many minerals so reflects the light as to 

 give it a peculiar luster. Thus quartz usually has a vitreous or 

 glassy luster, galena a metallic luster, selenite a pearly luster, 

 chalcedony a waxy luster. 



Then many minerals when scratched or, better still, when 

 rubbed on a piece of unglazed white porcelain yield a streak that 

 is peculiar. In this manner hematite is distinguished from 

 limonite, which it often resembles, for the former yields a red 

 streak, the latter a yellowish-brown one. 



