hard as a rock

Hard as a rock is a common saying which refers to something that is very hard since rocks are some of the hardest objects on Earth. Hard as a rock can be used more in a literal sense when speaking about an actual object that is very hard to the touch (but not really hard as an actual rock) or it can be used figuratively when discussing something that is hard or difficult to do. It is fairly typical to use the latter, where a situation is hard as a rock.