Avatar: The Last Airbender
Throughout "Avatar: The Last Airbender", Toph is the first earthbender known to bend metal. When Toph is imprisoned in iron, the sadhu Guru Pathik explains to Aang in a parallel scene that metal is refined earth; whereupon Toph locates the iron's impurities and manipulates them to "bend" the metal portion. She practices and hones her initially unique skill throughout the rest of the series, later teaching it to other earthbenders prior to the events of the "Legend of Korra".