BBB Etymology - Apple of one's eye

Originally meant the pupil of the eye, thought to be globular and solid like an apple, and acquired its present metaphorical sense because of the special preciousness of the pupil and the need to protect it. This sense is first found in the Bible in several places, e.g. 'he led him about, he instructed him, he kept him as the apple of his eye' (Deuteronomy, 32:10).