Pegmatite is not a stone in and of its self, but more a combination of several different type fussed together under extreme conditions.
Pegmatites are extreme igneous rocks. Igneous rocks are formed from the solidification of molten rock material that form during the final stage of a magma’s crystallization.
They are extreme because they contain exceptionally large crystals and they sometimes contain minerals that are rarely found in other types of rocks.
To be called a "pegmatite," a rock should be composed almost entirely of crystals that are at least one centimeter in diameter. The name "pegmatite" has nothing to do with the mineral composition of the rock.