Muscovite was once commonly used for windows, and the productive Russian mica mines gave muscovite its name (it was once widely known as "Muscovy glass"). Today mica windows are still used in cast-iron stoves, but the greater use of muscovite is as insulators in electrical equipment.
In any low-grade metamorphic rock, a glittery appearance is very often due to a mica mineral, either the white mica muscovite or the black mica biotite.