The diamond industry's Kimberley Process is failing due to a lack of accountability and follow-up, paving the way for an illegal trade that could see a return of conflict stones to world markets, campaigners said.

The certification scheme, designed to eliminate the trade in so-called "blood diamonds", was set up in 2003 in the wake of devastating civil wars in Angola, Sierra Leone, and Liberia, which were largely financed by the illicit diamond trade.

Before its implementation, conflict stones made up about 15 percent of the world market. Though they are believed to account for...