What are “conflict diamonds”?

They refer to the diamonds mined in poor African countries in the rebel-held areas.
The share of conflict diamonds has declined from 4% in the 1990’s to less than 1% now ever since the Kimberley Process Certification Scheme (KPCS) was implemented in 2003. As per the scheme, countries importing rough diamonds will only accept gem imports which are certified not to have come from the rebel-held areas.