From the rules:
[...]A defeat of a Major ‘6’ Insurgent in a diamond province [marked with a diamond: Katanga, Maniema, Kasai-Oriental or Kasai-Occidental] will result not only in a medal (or medals) being awarded, but also any player with a Peacekeeper in the province will receive a diamond. The diamond can either;
1. Be sold for $500 during a player’s turn
2. Be retained for extra medals at game end.