Well I always see that because Sapphire heard Sweetie sing one of Shores' songs "All the Single Fillies" (In my headcanon, Sapphire is the Beyonce of Equestria and is married to Neigh-z) telling Sweetie how impressed she was and how she can make it as a singer and as she walks away, sweetie gain he cutie mark: An eighth note with hearts at the end and if you look close at the hearts, there are little sapphires at the end.