It's a very good idea. Petunia is a paleontologist and fossils are essentially petrified bones, which makes them akin to rocks, and rocks are Maud's metier.

So Petunia has all the reasons to ask her for advice. I bet there's a fossil in this rock and Petunia is asking Maud what kind of rocks it exactly is, in order to find the right way/tool to break the rock and get the fossil out without destroying or damaging it.