When is a Stone Not a Stone?

If the littlest one got in your shoe you would feel it and say 'Stop, I've got a stone in my shoe', if I threw it at your head you would hardly feel it, but you know all about it if I threw the big one!
There must be a grading scale from gravel through pebble to stone to rock! So one of these must fit the bill!

Been busy, returning G's kilt to the hire shop, getting the car MOT'd (passed), editing Annie and John's photographs - so immersed in photographs I nearly forgot to take one today!