It is very unlikely and inconsiderate of Maud to not choose a rock as her coltfriend. Even she knows that rocks are sentimental. What if Boulder is actually heartbroken and jealous about her new coltfriend?

Although, Boulder does have Twiggy but what if they don't like each other in that way and instead like their owners?

(Btw, this is all a joke so don't take it seriously.)