Corrected entry: In the movie before this, Wishmaster 2, the girl has the stone in the end. In this movie it has mysteriously gone back in time and got itself into a Sumerian puzzle box that was buried five levels into a archealogical site that is protected on all sides. I don't think that an inner city girl has that kind of time on her hands.
Corrected entry: The angel Michael states that the sword of justice is the only thing that can kill the Djinn. Later on the Djinn is pinned up against a wall where he is in a very vulnerable position, and for some reason they forget to use the sword to kill him...
Correction: They don't use the sword because only Diana (A.J. Cook) can kill him and being that she isn't ready to handle the sword (She dropped it in the "test" earlier in the movie) They can't do it at that time.
Correction: There are several djinn and it's very possible that more than one stone was made to trap them. This is probably a different stone housing a different djinn than the one from the first two fims.