Biotite is so common that it's considered a rock-forming mineral. Biotite is named in honor of Jean Baptiste Biot, a French physicist who first described the optical effects in the mica minerals. Biotite actually is a range of black micas; depending on their iron content they range from eastonite through siderophyllite to phlogopite. If you aren't a mineral fanatic, ignore all that.
Biotite occurs widely throughout many different rock types, adding glitter to schist, "pepper" in salt-and-pepper granite, and darkness to sandstones. But it is the predominant mica in mafic rocks like gabbro. Biotite has no commercial uses and rarely occurs in collectable crystals. It is useful, though, in potassium-argon dating.
A rare rock occurs that consists entirely of biotite. By the rules of nomenclature it is called biotitite, but it also has the fine name glimmerite.
