Black Diamond is a rare item in Dokapon Kingdom which can be given to the king or sold for 45,000G. If given to the king, it will have a large negative affect on the giver's Local Item Value. Because of this effect, it is very beneficial to send this item at a Temple under another adventurer's name.
It has a 40% drop rate from Rico Jr. after defeating him in his initial boss fight. In random encounters with him afterwards, it has a 12.5% drop rate.