The Democratic Republic of Congo has one of the richest deposits of such metals, however, the African country has been torn by a civil war for several years and local chieftains have made a fortune by exploiting the local population to mine precious ores. The mined tantalum, tungsten, or gold are then sold.
The Cupertino giant also disclosed a list of the local smelters and refiners that cooperate with the company in order to be “clear which ones have been verified as conflict-free.
Apple's report also states that the company has educated over 1.5 million in their social rights and provided free education and development courses for more than 280,000 of its workers worldwide in 2013. In addition, 33 specialized audits have been carried out in order to ensure that migrant workers, working at facilities with “unfair treatment”, are safe. Work hours in most of the factories have also been limited and adjusted to Apple's international work standards.
Back in 2011, Apple was accused that it failed to enforce its safety standards within some of its Chinese suppliers, which reportedly led to the poisoning of several workers.
source: Apple via MacRumors