Whenever I get any angels I usually sculpt little devil horns and a tail on them. I just put the polymer clay directly on the figure then bake according to the directions. As long as the angel isn't plastic, it should be ok in the oven. Then i paint it like stone.
Similar to Frankie's girl suggestion you could age them with paint. I start by painting the statue black. I then drybrush white or grey. Finally, you can also drybrush a little moss green color in spots to make it look like lichen or mildew. One other subtle suggestion is to paint the pupils of the eyes white. It can give the impression the statue is looking at you. Good luck.