tool for what? that is the question you should always start with. your rock is a corner that just got knocked off at one point. "fits in your hand," is usually a give-away for not-an-artifact too.
Hey Slycer those rocks on full flow tumble running into each other can actually cause damage like the one you found. Keep looking but look for flaking removed on the next find. Just natural this time.
I'm afraid it's just a rock as already mentioned. Most anything fits the amazing human hand, a rock, toothbrush, remote, spoon, hammer, broom, steering wheel, coffee cup...