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.
- He built his house out of stones.
is as acceptable as
- He built his house out of rocks.