Wiki defines found art as an art which, “...derives its identity as art from the designation placed upon it by the artist...” Sure, the best manmade rockart has a wow factor that simpler manmade or natural cup and rings don’t but that doesn’t preclude them from being a form of art (if we can even use that term here). Where would you place the Alphamstone stuff? Probably natural but found on a Christianised site and perhaps brought and incorporated there because they looked ‘artificial’ (manmade or otherwise) or at the very least looked interesting and different enough for use at an important site. Ditto the ammonite at Stoney Littleton, the holed stones down by the river at Pewsey, the twisted stones used at Rollright, the puddingstones used up in this (Essex) neck of the woods.
Sorry if I’ve got hold of the wrong end of the stick – just wanted to say that art doesn’t have to be manmade but that it becomes art when we make it so.