Maimonides on Mishley
A word fitly spoken is like apples of gold in vessels of silver. (Prov. 25:11)
Hear the explanation of what he said; the word maskiyoth, the Hebrew equivalent for vessels denotes filigree network, i.e., things in which there are very small apertures, such as are frequently wrought by silversmiths. They are called in Hebrew maskiyoth (lit. transpicuous from the verb sakah, he saw a root which occurs also in the Targum of Onkelos, Gen. 26:8) because the eye penetrates through them. Thus Solomon meant to say, just as apples of gold in silver filigree with small apertures, so is a word fitly spoken.
See how beautifully the conditions of a good simile are described in this figure! It shows that in every word which has a double sense, a literal one and a figurative one, the plain meaning must be as valuable as silver, and the hidden meaning still more precious: so that the figurative meaning bears the same relation to the literal one as gold to silver. It is further necessary that the plain sense of the phrase shall give to those who consider it some notion of that which the figure represents. Just as a golden apple overlaid with a network of silver, when seen at a distance, or looked at superficially, is mistaken for a silver apple, but when a keen-sighted person looks at the object well, he will find what is within, and see that the apple is gold. Maimonides intro to the Guide