I like to think that the rock is like tsukasa , we don't know exactly who she is or if she might have come from an earlier time or been alive for awhile, but she is still a girl and nasas wife. Just like granny said its just a rock, even if it took thousands of years to get its still just a rock. Tsukasa may be mysterious but she is still just a girl that wants normal human things, so I think that the granny was hinting that by saying make her happy, don't treat her any differently then a normal girl.