COLOR OF MINERALS. 57 



Sulphur-yellow, straw-yellow, wax-yellow, ochre-yellow, 

 honey-yellow, orange-yellow ; 



Scarlet-red, blood-red, flesh-red, brick-red, hyacinth-red, 

 rose-red, cherry-red ;y 



Hair-brown, reddish-brown, chesnut-brown, yellowish- 

 brown, pinchbeck-brown, WOod-brown. 



A flay of colors : this expression is used when several 

 prismatic colors appear in rapid succession on turning the 

 mineral. The diamond is a striking example ; also precious 

 opal. 



Change of colors : when the colors change slowly on turn- 

 ing in different positions, as in labradorite. 



Opalescence : when there is a milky or pearly reflection 

 from the interior of a specimen, as in some opals, and in 

 cat's eye. 



Iridescence : when prismatic colors are seen within a 

 crystal ; it is the effect of fracture, and is common in 

 quartz. 



Tarnish : when the surface colors differ from the interior ; 

 it is the result of exposure. The tarnish is described as 

 irised, when it has the hues of the rainbow. . 



Pleochroism .** the property, belonging tosorrie prismatic 

 crystals, of presenting a different color in different directions 

 The term dichroismf has been generally used, and implies 

 different colors in two directions, as inrthe mineral iolite, 

 which has been named dichroite beo&use of the different 

 colors presented by the bases and sides of the prism. Mica 

 is another example of the same. The more general term has 

 been introduced, because a different shade of color has been 

 observed in more than two directions. 



These different colors are/observed only in crystals with 

 unequal axes. The colors/ are the same in the direction of 

 vqual axes,, and often unlike in the direction of unequal axes. 

 This is the general principle at the basis of pleochroism. 



What is a play of colors 1 change of colors 1 opalescence ? irides* 

 ence ? tarnish ,? dichroism and pleochroism 1 Mention examples of 

 tiis last property ; also the law relating to it. 



*" From the Greek pleos, foil, and chroa, color. 

 frFrom the Greek dis, twice, and chroa. 



