Conflict diamonds, also known as blood diamonds, are diamonds harvested as the product of illegal labor. Many are from Northern Africa. Farmers force citizens to mine diamonds with no income and sell them for a large profit. The Kimberly Process is a method designed to prevent illegal harvesters from profiting off of conflict diamonds. There are 81 countries participating in this cause. Sellers are required to show ID as well as proof of valid mining. The Kimberly Process has been effective, but there are still a large number of illegal farmers profiting. This is why there is so much controversial criticism around the topic.
(Originally written: Information from official KP website and prior knowledge)