72 PHYSICAL PROPERTIES OF MINERALS. 



Haii-brown, reddish-brown, chestnut-brown, yellowish-brown, 

 pinchbeck-brown, wood-brown. 



A play of colors — this expression is used when several pris- 

 matic 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 turning 

 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. 



2arnish — 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. 



2. Dichroism, l^richroism. — Some crystals, under each of the 

 systems excepting the isometric, have the property of present- 

 ing different colors by transmitted light in different directions. 

 The property is called dichroism when these colors are seen in 

 two directions, and trichroism (or pleorhroisni) if seen in three 

 directions. The colors are always the same in the direction 

 of equal axes and often unlike in the direction of unequal 

 axes. As dimetric and hexagonal crystals have the lateral axes 

 equal they can present different colors only in two directions, 

 the vertical and lateral; while all crystals that are optically 

 biaxial may be trichroic. 



The mineral iolite is a noted example, and received the name 

 dichroile on account of this property. Transparent colored 

 crystals of tourmaline, topaz, epidote, mica, diaspore, and many 

 other species exhibit it. Tourmaline crystals, when transpar- 

 ent or translucent transverse to the prism, are opaque in the 

 direction of the vertical axis ; and so also are thick crystals of 

 mica. Colored varieties of hornblende are dichroic, while 

 those of the related mineral, pyroxene, are not so. 



This quality is best observed by means of polarized light. On 

 examining a mineral with a tourmaline plate, or Nicol prism, the 

 two colors in a dichroic mineral are successively seen as the 

 tourmaline or Nicol is revolved ; and if there is no dichroism 

 there is no change of color. A small instrument, containing a 

 prism of calcite, has been constructed for showing the dichro- 

 ism, called the dicliroscope. On looking through it at a di- 

 chroic crystal, the aperture against the crystal appears double, 

 owing to the double refraction of the calcite, one image being 

 made by the ordinary ray and the other by the extraordinary 



