contrast

Act 4.1.29-35
There is a clear contrast between the Duchess (light) and Ferdinand (dark) throughout the fourth act. While the duchess shuns the dark that is forced to surround her (coming from Ferdinand's attempts to control her), Ferdinand seems to represent the dark. He is very cold (as even stated by the Duchess) and cruel. However, he is completely evil/dark considering he has a conscious that was upset after her death. Perhaps this is why there are references to diamonds; a thing of value which the duchess is supposed to represent, yet deteriorating by the bad influences of her brothers.