Goldenheart

Goldenheart is a kind of tree that grows in the Summer Islands. The Summer Islanders use it to make bows which are renowned for their great strength, capable of launching a shaft of hardwood with such force that it can penetrate steel plate armor.

In the books
Goldenheart wood is renowned for its strength, and makes the best bows in the world - save only for dragonbone, which is far more rare (and often only found on archaeological digs around Asshai, etc.) and not renewable.

Goldenheart is therefore highly sought after, but the Summer Islanders themselves have a strict ban on any export of it - instead reserving it for their own armies and sailors. This restriction was put into place thousands of years ago after a series of wars against foreign slavers and pirates. Nonetheless some goldenheart is smuggled out now and again and is used to make a few rare objects in the Free Cities or Westeros. During the Tourney of the Hand in the first novel, Jaime Lannister himself rides in the joust using a splendid lance made out of goldenheart wood.

There are three major isles in the Summer Islands, from north to south: Walano, Omburu, and Jhala. Walano is the most populous, while Jhala is the largest, as big as the other two put together. Goldenheart doesn't grow on Walano, however, only on Omburu and Jhala.