How very confusing, the decorative pieces of geology in our gardens (UK) are called "rocks" and the bits of geology on my drive way is called shingle (sort of like gravel) but shingle can be graded by size......  I would put stones in my fish tank (if I had one).  A boulder is something where we can agree.......... we find them on mountains too!

 The term "shingle"  here is used as a material that covers your roof top...or could also be a medical condition.