Silver

No element conducts electricity or heat better than silver, and none reflects visible light more efficiently, which is why it is used where cost is no object: RF contacts, brazing alloys, mirror coatings and the conductive paste that carries current off solar cells, now around a tenth of all demand. For a century and a half its defining use was photography — silver halide crystals darken where light has struck them, and the whole industry ran on that reaction until digital sensors ended it in about a decade. The Potosí mine in Bolivia, worked from 1545, poured so much silver into Europe that it drove inflation across the continent and financed the Spanish Empire. Silver tarnish is not oxide but silver sulfide, formed from trace hydrogen sulfide in air, which is why eggs and rubber bands blacken silverware so quickly.