So.. I put my cafe on a place where a rock used to be, even though Isabelle told me the placement was good.
This resulted into a rock being in a different spot every day. Is there any way to fix this except from restarting?
I don't know, the first few times she told me that it was too close to a rock, so the cafe couldn't be built there, but then I moved a few spaces and then she said that the place was perfect, but my rock disappeared nevertheless.
No Kaiaa is right, it was your gem rock. It's a fake rock that spawns in a new spot everyday. You can get gems out of a regular rock with the silver shovel, though, which might be what you're thinking of?