One of the most moving metaphors in the movie is the "rough gem". Grandpa Nishi shows Shizuku a geode—ugly on the outside, but hiding brilliant crystals within. He tells her:
This is the whisper of the heart. It is not the shout of genius. It is the quiet, persistent murmur that tells you to keep going, even when the result is garbage. It is the courage to be a beginner. Whisper of the Heart