Huh...I've noticed something. There are a preset number of "types" of a floor. And based on the type of floor you're on, you can predict what will be in the rocks (and potentially what will be hoed/what each crack will do?). For example, in the "default" position for the second floor of Moonlight Mine (this is where the stairs on the first floor are on the lefthand side; I think this is always where they are on the first visit of the day), the upper-right rock that stands alone in its 3x3 always has limestone, and the rocks will always spawn the same stuff.
I know this board is all but dead, but ramblings ensue. I'm sure if someone really wanted to, there could be a GREAT guide developed for the mine.