What's the difference between "rock" and "stone"?

What's the difference between "rock" and "stone"?

A rock is bigger, possibly immovable, you couldn't throw it with one hand, at a pinch you might be able to with two.

Above, a Google Image result for "Rock", below, for "Stone"

Note that a boulder is probably bigger than a rock¹.

Below, a boulder

Oh, also note that rocks and boulders are made from stone.

(This answer is mildly tongue in cheek)

¹ Notable exception: Uluru/Ayre's Rock.

Generally, usage has made the two words completely interchangeable. However, I would use stone to refer to the stuff from which rocks are made.

• His house is made of stone.

is probably better than

• His house is made of rock.

But

• He built his house out of stones.

is as acceptable as

• He built his house out of rocks.