Coltan

Coltan is used for the production of tantalum capacitors.

[…] as of 2003 over one billion cell phones were in use worldwide, so by the time the high-tech bubble approached its bursting point in 2000 and 2001, coltan had become an extremely hot commodity.

Consider your mobile phone. Before it was assembled in a Chinese factory, the coltan in its capacitors may have been dug by miners in the Eastern Congo, where millions have died in a series of wars over ‘conflict minerals’, though we give this no more thought than previous generations of Westerners gave to the Congolese origins of the ivory in their piano keys, the rubber in their tyres, the copper in their bullet casings or the uranium in their bombs.