Tantalum, Tin, Tungsten and Gold are referred to as Conflict Minerals. Armed rebel groups and militia, most notably in the eastern provinces of the Democratic Republic of Congo (DRC), sometimes fund their violent and oppressive activities with money obtained from the sale of these minerals.

Links:
Conflict Minerals Statement
2016 Conflict Minerals Report
2016 Smelter List
2016 Supply Chain Due Diligence Summary