Possibly a rock called Diorite. Minerals contained, muscovite and or biotite (black and or white mica); plagioclase - a sodium or calcium feldspar - most of the lighter material in the rock; and hornblende, another calcium/sodium silicate, chemically somewhat similar to plagioclase but blackish in color. It could a different type of rock but based on the pic this would be initial guess. No real commercial value. Rounded from bouncing down a stream or river.
Hope this helps.
3xflyfisher