Pleochroism in Tourmaline
Tourmaline is a pleochroic mineral. It varies in color depending upon the direction of observation. If you have a tourmaline crystal the color will generally be darkest looking down the long axis of the crystal and lighter when you view the crystal from the side.
In this video we rotate a blue tourmaline crystal segment and a yellow tourmaline crystal segment. Not how they are darkest when you view the crystal cross sections and lighter when you view the striated sides of the crystal.