Muscovite

Muscovite is a common primary mineral in many rock types: it is especially common in granitic rocks, forms large "books" in granite pegmatites, and grows during regional metamorphism of clay-rich rocks into mica schist and gneiss. In the 1700s sheets were mined from pegmatites around Moscow, Russia. The retailer Pascle lists Russia and China as sources of its red-brown beads.

Muscovite is identified by its perfect cleavage into thin, flexible, elastic, nearly colorless transparent sheets with a pearly to vitreous luster; Geology.com calls it the only common mineral with these properties. It may be confused with margarite and with lithium micas; a red or pink color alone does not prove lithium (lepidolite), so pink muscovite and lepidolite cannot be told apart by color alone. Green chromium-bearing muscovite is sold as fuchsite.

Early names for muscovite include Muscovy glass, cat silver and lapis specularis ("stone mirror"), recorded in seventeenth-century and earlier texts. Its ability to split into large transparent sheets gave it an early use as window panes; panes mined near Moscow were called "Muscovy glass", the term thought to have inspired the name muscovite, which Johann Gottfried Schmeisser used by 1794.