She is always also bugging me about how our house is going to be more cheerful than she would like and how I spend way too much money on animals that you cant hold.
Slate could be shaped to look gravestone-ish, sand the edges so they aren't sharp. Comes in gray, black.
Tree like wood with just tufts of moss could be creepy. Or staghorn algae would be cool if you could control it, lol, scary in more than 1 way.