The Iron Pillar of Delhi
375–415 CE
A 1,600-year-old iron pillar standing in the Qutb complex in Delhi has not rusted despite being exposed to the elements for sixteen centuries. Modern metallurgical analysis revealed it is made from 99.72% pure wrought iron with a thin protective layer of misawite — an iron-hydrogen-phosphate compound that forms a self-healing rust barrier. The technology to produce iron of this purity was not achieved in the West until the 19th century.