What object from Earth grew into the Narnian lamp-post?

A piece of iron from a London lamppost grew into Narnia’s famous lamp-post.

In The Magician’s Nephew, the witch Jadis tears an iron bar from a lamppost in London during a violent confrontation. She carries it through the magical passage between worlds and later throws it at Aslan when she encounters him at the creation of Narnia.

The bar does not harm Aslan. Instead, it falls onto Narnia’s extraordinarily fertile new soil and begins to grow like a plant. It becomes a working lamp-post, complete with a light, in the area later known as Lantern Waste.

This origin explains the apparently mysterious lamp-post that Lucy encounters when she first enters Narnia through the wardrobe. The answer is not a sword, key or ordinary stone: it is specifically an iron bar or piece torn from an Earth lamppost. The scene also turns an object associated with violence into a lasting landmark of Narnia.