Well, that has the exact size and shape of the perforated whetstone illustrated in Curtis Hoffman's "A Handbook of Indian Artifacts from Southern New England"(1991, MAS, p.71) The preferred materials for such whetstones are argillite(slate), sandstone, and schist. If it's a whetstone, you should notice "wear from long even lengthwise strokes of rubbing. Because of this, and the narrow proportions of this honing stone, it is thought to have been used in grinding the blades of gouges and other woodcutting implements."(Hoffman, p. 71). Also, when whetstones are perforated, the perforation is fashioned by countersunk pecking on both sides first, and then finished by drilling. I've seen the additional photos you posted on another site, and that's the way your piece was perforated. So I'm guessing whetstone. With all this being said, it certainly resembles a large pendant as well.
