pleochroic

“Tourmaline hornfelses” are found sometimes near the margins of tourmaline granites; they are black with small needles of schorl which under the microscope are dark brown and richly pleochroic.

The eye of the observer placed at e will see two images of the square aperture, and if a pleochroic crystal be placed in front of this aperture the two images will be differently coloured.

Hypersthene, when present, is often strikingly pleochroic in colours varying from pink to bright green.

When polarized light is passed through a transparent plate of a pleochroic mineral, the colour will vary with the direction in which the light-vibrations take place.

The pleochroism is strong, the pleochroic colours being usually green, yellow and brown.