Lace rock is definitely nice for building, and like mojoreef's mine is covered in corraline and tiny feather dusters. I know the stuff I get is natural.
I know some people have experimented with rock made from concrete, which of course would mean you could make it any shape you want. From what I've read, if you don't cure and clean it right it can really cause problems. That may be what DonW has seen happen.