Such an effect is called pleochroism, and crystals which show variations in their colour when viewed from different angles, or by transmitted light, are called pleochroic, or pleochromatic -- from two Greek words signifying "to colour more.".    From Wordnik.com.  [The Chemistry, Properties and Tests of Precious Stones] Reference

Biotite containing zircons surrounded by pleochroic.    From Wordnik.com.  [WN.com - Articles related to Obama to announce nuke plant loan] Reference

Under the microscope some of the biotite has a green coloration from decomposition and is surrounded by strong pleochroic halos.    From Wordnik.com.  [The Long Labrador Trail] Reference

1986, "Gentry's pleochroic halos: Creation/Evolution," Newsletter, Feser, Karl D.    From Wordnik.com.  [Original Signal - Transmitting Digg] Reference