The loop half of key is an item dropped by any monster that has access to the rare drop table. It is also a reward from various random events, and can be found in the chest in Taverley, the gnome restaurant, from Kingdom Management and from caskets. It can also be found in the Barrows chest as a separate item with 100% rewards potential.
The loop half of a key is used with a tooth half of key to make a crystal key. Doing so will grant you 10 Crafting experience. This is used to unlock the crystal chest, which contains a dragonstone, and other items.
- ↑ Jagex. Mod Kieren's Twitter account. 3 Mar 2016. Mod Kieren: "You guys wanted info on Barrows loot, here it is... Full size:  Explained the best I can!"