The earrings are made from the Evil Black Crystal itself, they are diamond-shaped. The leaders of the Black Moon Clan, with the exception of Saphir, wore these earrings. They are signs of loyalty to Nemesis and symbols of power.
In Act 23, Prince Demande and Saphir received more powerful crystal earrings from Black Lady, to insure their obedience to Wiseman. These new earring would also allow them to travel freely through Wiseman's Castle of Darkness.