Apple of your eye

Originally meaning the central aperture of the eye it is more popular in its figurative sense. 

'The apple of my eye' is exceedingly old and first appears in Old English in a work attributed to King Aelfred (the Great) of Wessex, AD 885, titled Gregory's Pastoral Care.

Much later, Shakespeare used the phrase in A Midsummer Night’s Dream, 1600:

Flower of this purple dye,

Hit with Cupid’s archery,

Sink in apple of his eye

It also appears several times in the Bible; for example, in Deuteronomy 32:10 (King James Version, 1611)

He found him in a desert land, and in the waste howling wilderness; he led him about, he instructed him, he kept him as the apple of his eye.

and in Zechariah 2:8:

For thus saith the LORD of hosts; After the glory hath he sent me unto the nations which spoiled you: for he that toucheth you toucheth the apple of his eye.

The phrase was known from those early sources but became more widely used in the general population when Sir Walter Scott included it in the popular novel Old Mortality, 1816:

"Poor Richard was to me as an eldest son, the apple of my eye."

(source)