Crystal Healing

I collected some rocks from a tertiary marine sediment bank exposed along a roadside in mid-Virginia near Richmond ( USA), and picked out some really varied rocks, from a greenish crystal type, to some layered sedimentary types, to this round, smooth black rock. Quite a variation in rock types. Any idea what the all-black one might be? Doesn't appear to be obsidian, I did a little Dremeling with a diamond bit, didn't polish up as much as I thought, seemed soft. Anyway, just would like some thoughts on what mineral type it might be.