Pound Coin of Shakespeare's time, showing Elizabeth I
Hamlet says to the boy who plays women's parts, "Pray God, your voice, like a piece of uncurrent gold, be not cracked within the ring." Coins were typically stamped with a sovereign's head inside a circle; if a coin had a crack from the edge into the circle (was "cracked within the ring") it was "uncurrent," not lawful currency. Hamlet is punning on the word "cracked" because when a boy's voice cracks—changes from boyish treble to adult baritone—that boy can no longer play women's parts.
Pound Coin of Shakespeare's time, showing Elizabeth I