CHRIS S.........Volcanic rock idea pretty good, but, if ppl in your area are good at looking for "things out of place" then a real porus redish rock amongst smooth grey (or what ever the type) would stick out like a sore thumb, if they are not so good, would be a good "container" hehe!
(a grey or white flat paint, or combo of might help it blend in more)
And your right, it drills out really easy, (especially with a concrete/mortor bit) chisel werk tad tough and you run risk of splitting the rock...................