The world’s biggest diamond producer hired an external investigator to find the source of possible leaks of its most sensitive price data, according to people familiar with the matter. De Beers hired investigators from KPMG International in South Africa to find the source of the leaks, according to the people, who asked not to be identified because the matter is private. The company suspected at least one employee sold pricing information to customers before the diamonds were offered for purchase, the people said.