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 ; 



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

 brown, pinchbeck-brown, wood-brown. 



A play 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 to some prismatic 

 crystals, of presenting a different color in different directions 

 The term dichroism-f has been generally used, and implies 

 different colors in two directions, as in the mineral iolite, 

 which has been named dichroite because 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 

 equal ayes, 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 ? change of colors ? opalescence ? irides- 

 oence ? tarnish ? dichroism and pleochroism ] Mention examples of 

 this last property ; also the law relating to it. 



* From the Greek pleos, full, and chroa, color. 

 rFrom the Greek dis„ twice, and chroa. 



