Ashok Pillar of Delhi has baffled scientists as it weathers all vagaries of weather and yet does not rust or corrode. Which is it made of?

The Iron Pillar at the Qutub complex, Delhi, dating from the Gupta period (about 4th–5th century AD), is made of wrought iron. A thin protective layer of iron hydrogen phosphate, helped by its high phosphorus content, keeps it from rusting.