The Collaborative International Dictionary of English v.0.48:

Greenstone \Green"stone`\ (gr[=e]n"st[=o]n`), n. [So called from
   a tinge of green in the color.] (Geol.)
   A name formerly applied rather loosely to certain
   dark-colored igneous rocks, including diorite, diabase, etc.
   [1913 Webster]