It's hard to mold a Buddha's bones, but you don't know what it means to know your face but your heart.
It's hard to really understand the heart and essence. The phrase "it is difficult to shape a Buddha, but it is difficult to shape a bone" means that when shaping a Buddha, the shape is easy to shape, but it is difficult to shape the essence and charm of the Buddha. It is difficult to really understand a person's heart and essence when you know him, although you can understand the superficial features through his appearance and words and deeds.