According to http://www.bloomberg.com,

De Beers lowered prices by about 3 percent, with bigger reductions for certain categories, at a sale this week in Botswana, according to the people, who asked not to be identified as the information isn’t public. The miner offered about $450 million of rough diamonds at the sale, the people said.

Full story here: http://www.bloomberg.com/news/articles/2015-05-06/de-beers-said-to-cut-diamond-prices-in-surprise-blow-to-anglo