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.
I'm not sure that I would ever refer to "a rock". To me, "rock" is a substance - I recognise "a rock" but think of it as a principally American word. I do talk about "a stone" as well as "stone" - "stone" is the substance which has been, or may be, used for building.
Generally, usage has made the two words completely interchangeable. However, I would use stone to refer to the stuff from which rocks are made.
is probably better than
is as acceptable as
protected by RegDwigнt♦ Feb 24 '12 at 11:01
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