This anecdote is actually taken straight from a Catholic Catechism by Joseph Baxter.

A certain monarch caused the figure of an angel to be carved in white marble. From the left hand of this statue hung a silver ring attached to a thin silken cord, while the right hand held a golden ring suspended from a diamond chain. The king's son and daughter asked their father what these two rings were intended to signify. He answered: "I will give the rings to whichever of you can guess their meaning aright." Then the prince said: " The rings are doubtless emblems of friendship and love." The king replied: " That is quite right. But why is one ring made of gold and the other of silver?" The princess answered: "The silver ring signifies human friendship and affection. That friendship, that affection, cannot be relied upon; it hangs, as it were, by a slight cord which is easily broken. The gold ring signifies the love of God for man ; that is firm and unchangeable ; it cannot be broken." The king praised his children for the good answers they had given ; he gave the silver ring with the silken cord to the prince, and the gold ring with the chain of diamonds to the princess.

On promise rings see:
Promise rings: history and meaning