rock / stone

Two main versions of this sign:
Both versions of stone/rock are interchangeable.
Sort of like saying "sofa" and "couch."
Or "pop" and "soda."
They are the same thing and different people prefer one or the other..

ROCK:
Whack the back of the palm-down left fist with the palm down right fist.

STONE:
This sign can also be used to mean "rock" but I use this more for "concrete." Note: I've seen some people sign "tooth" to mean concrete or ceramic.
Use an "S" hand. Place it under the chin and bring it forward twice. The front of your fist brushes up against the bottom of your chin and slides forward an inch or two.

For additional information, see MOUNTAIN