The main RuneScape Wiki has an article on: Half of a key
The Old School RuneScape Wiki also has an article on Half of a key.

A Half of a key is a rare item drop which may be received from any monster that drops gems. Using the "teeth" half on the "loop" half or vice-versa will create a crystal key. Using said key on the Crystal chest in Taverley will give the player several rare items. The chest has a 100% chance of giving an uncut dragonstone, as well as the possibility of various other items, such as coins, runes, and rune bars.

Dropped by

They are dropped by:

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