Muscovite mica derives its name from it's usage of "Muscovy" glass as glass panes many years ago. The primary source of Muscovite mica today is India. Natural mica in raw (also called Block) form is classified by how many square inches of usable area can be obtain per piece. Grade 4 block will yield a usable area of 6 to 10 square inches. Colors can range from a pale ruby to light green. Color depends on which area it's mined and will vary from lot to lot.