Bird's eye maple

Bird's eye maple is a specific type of extinction exhibited by micas, especially muscovite under cross polarized light. It gives the mineral a pebbly appearance as it passes into extinction. This is caused when the grinding tools used to create petrographic slides of precise widths alter the alignment of the previously perfect basal cleavage planes which split micas up into its characteristic thin sheets. The resulting, slightly roughened surface alters the extinction angle of various parts of the crystal lattice, leading to this type of extinction. Since it is not a natural feature of the mineral, bird's eye maple is not observed in all mica crystals, nor from all angles, but it is quite common, and is used as a diagnostic feature for micas.