Conflict diamonds are diamonds that have been mined from a region that uses the sale or trade of diamonds to support military action, western and central African being one of the worst known culprits.
The United Nations (UN) describes conflict diamonds or 'blood diamonds' as:
"Diamonds that originate from areas controlled by forces or factions opposed to legitimate and internationally recognized governments, and are used to fund military action in opposition to those governments, or in contravention of the decisions of the Security Council."
Diamond Facts. Org, Conflict Diamonds, 2013