An optical study is made of the etch pits produced by hydrofluoric acid on the cleavages of muscovite mica. Two types of etch pit are found (a) small widely distributed pits and (b) relatively large localized isolated pits. On oppositely matched cleavages there is exact one-to-one correspondence between the localization and shapes of the isolated pits. These are studied both with a light-profile microscope and interferometry. It is established that cleavage lines are displaced by the etching, and this is discussed. The mechanism of the formation of the isolated pits is discussed and they are attributed to impurities. On occasional samples winged pits are formed and the wing is shown to be an air wedge within the body of the mica.