Tantalum

Anodise tantalum and it grows an amorphous Ta₂O₅ film a few tens of nanometres thick with a high dielectric constant and the ability to repair its own pinholes — which is how you fit a lot of capacitance into a component the size of a grain of rice. That property put tantalum capacitors into essentially every mobile phone, laptop and hearing aid. The same immunity to corrosion, including immunity to aqua regia, makes tantalum the metal of choice for chemical reactor linings and for surgical implants, since body tissue tolerates it completely. Its main ore, coltan, is mined heavily in the eastern Democratic Republic of the Congo, where the trade has financed armed groups for decades and prompted the conflict-minerals provisions of the Dodd-Frank Act.