I have fake boulders in my little yard, though we did not make it. Just bought it at OSH, not cheap though! We decided to use it to cover up the protruding pipes. Some of my friends thought it was real, till they do a little knock and hear the hollow core. See the photos here:
fake boulder beside the succulents
click on the photo and the fake boulder is on the upper left side
So far we have had these since 2008, still looks okay, but I guess later on we may need to spray paint a bit. The weather wear and tear adds distressed look to it, and makes it more natural looking I think.
