Cobalt

Miners were puzzled at some varieties of copper ore which didn't yield copper when smelted, and suspected foul play from kobolds, a type of mythical sprite. Upon closer examination, it was a new element (after Phosphorus), and Brandt named it cobalt. This was the first metal to be discovered not known to the medeival alchemists.
 Georg Brandt Sweden 1737 Cobalt

Miners were puzzled at some varieties of copper ore which didn't yield copper when smelted, and suspected foul play from kobolds, a type of mythical sprite. Upon closer examination, it was a new element (after Phosphorus), and Brandt named it cobalt. This was the first metal to be discovered not known to the medeival alchemists.