labradorite for MM
Labradorite is a feldspar mineral. Labradorite can display an iridescent optical effect (or schiller) known as labradorescence. The term labradoresence was coined by Ove Balthasar Bøggild, who defined it (labradorization) as follows:
Labradorization is the peculiar reflection of the light from submicroscopical planes orientated in one direction (rarely in two directions); these planes have never such a position that they can be expressed by simple indices, and they are not directly visible under the microscope.