WMG / Natsuiro Kiseki

Rin and Natsumi's mothers used to wish on the big rock.

Rin's mom is the one that told the girls about the big rock in the first place, and she appears to know more than she lets on in the final episode. When we see her talking with Natsumi's mom, the scene implies that they've known each other for a while, and they specifically reference their past summer vacations.