Some minerals selectively absorb certain wavelengths of light, and the combination of wavelengths that pass through can give the mineral colour observable in plane polarized light.  If the selective absorption differs with orientation of the crystal, the absorption colours change with orientation.  This phenomenon is termed pleochroism.  Unless the mineral is oriented with an optic axis in the vertical position, pleochroic minerals change colour as the mineral is rotated in plane polarized light.  Uniaxial minerals can have two pleochroic colours, and biaxial minerals can have three, corresponding to the principal vibration directions of the mineral.  Images A and B (rotated 90 degrees relative to one another) show grains of biotite that illustrate pleochroism in shades of pale yellow-brown (dominant in A) to red-brown (dominant in B).  In C and D (rotated 90 degrees relative to one another), hornblende grains in a variety of orientations show three different colours (emerald green, olivine green/brown, and pale yellow), which differ for individual grains in the two images.  If only the percent of light transmitted varies with orientation, and not the colour (a function of the combination of wavelengths transmitted), the colour remains the same during rotation of the grain, but its intensity changes, and one sees darker or lighter versions of the same colour.  This is type of absorption is not true pleochroism, and is a property of the mineral tourmaline.  All fields of view are 2.2 mm across and in ppl.