Well, what I would be wary of is anything that just "looks like" something. If you were SURE it was limestone, and were relatively sure it was a clean area...I wouldn't have too many issues.
The "holey" rock commonly used is often found througout Texas in fields and such. So it is a similar situation. Limestone is not an issue...it is basically an old fossilized coral reef anyway. But if you aren't SURE of what it is, then I wouldn't use it.