Rock, stone, or pebble?

I’ve been running quite a few primary school rock and fossil classes in Yorkshire this half-term. At Crofton Junior School near Wakefield, one of the girls in the class asked an excellent question:

“What’s the difference between a rock, a stone, and a pebble?”

I started with the easy bit, which is ‘pebble’ as that’s a particle between 4mm and 64mm in size. As for rock versus stone, I decided that ‘rock’ was commonly used as a term for the natural material, and ‘stone’ for the material once it was utilized by people. It’s not a very satisfactory distinction though!

Have you got a better definition?