Muscovite (also known as common mica, isinglass, or potash mica) is a phyllosilicate mineral of aluminum and potassium with formula KAl2(AlSi3O10)(F,OH)2, or (KF)2(Al2O3)3(SiO2)6(H2O). It has a highly perfect basal cleavage yielding remarkably thin lamina (sheets) which are often highly elastic. Sheets of muscovite 5 m × 3 m have been found in Nellore, India.
Muscovite with beryl (var. morganatic) from Paprok, Afghanistan (dimensions: 5.9×4.8×3.4 cm)
Muscovite has a Mohs hardness of 2–2.25 parallel to the  face, 4 perpendicular to the  and a specific of 2.76–3. It can be colorless or tinted through grays, browns, greens, yellows, or (rarely)violet or red, and can be transparent or translucent. It is anisotropic and has high birefringence. Its crystal system is monoclinic. The green, chromium-rich variety is called fuchsite; mariposite is also a chromium-rich type of muscovite.
Muscovite is the most common mica, found in granites, pegmatites, gneisses, and schists, and as a contact metamorphic or as a secondary mineral resulting from the alteration of topaz, feldspar, kyanite, etc. In pegmatites, it is often found in immense sheets that are commercially valuable. Muscovite is in demand for the manufacture of fireproofing and insulating materials and to some extent as a lubricant.
The name muscovite comes from Muscovy-glass, a name given to the mineral in Elizabethan England due to its use in medieval Russia as a cheaper alternative to glass in windows. This usage became widely known in England during the sixteenth century with its first mention appearing in letters by George Tuberville, the secretary of England's ambassador to the Russian tsar Ivan the Terrible, in 1568.