The United Nations (UN) defines conflict 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."
The mining of these precious stones brings great benefits to the people in countries where they are sourced. But this same resource has in the past been used to fund conflict and crime. Sadly it is still occurring albeit in a greatly reduced capacity.
In 2000, a coalition of governments, non-governmental organizations and the diamond industry worked together to address this issue and in 2002, they established the Kimberley Process Certification System - a UN-backed process that has virtually eliminated the trade in conflict diamonds.
Today 74 governments have enshrined into their national law the Kimberley Process Certification System, resulting in over 99% of the world's diamonds being now sourced from conflict free areas.
Respected jewelers have written policies on this topic so always check for these.