The ruby key is a Precursor artifact seen in Jak II. Mar, the founder of Haven City, created the key himself which is able to unlock ancient doors in the Underport that lead to the outside beyond the city wall. Jak and Daxter retrieved the key for Krew during the "Drain sewers to find statue" mission after lowering the water levels in the Haven sewers, where they found the statue of Mar holding the key. Daxter climbed up the statue, and, with some effort, pulled the key from Mar's stone grip.
Krew wanted the key so he could unlock ancient passageways in the Underport to let the Metal Heads into the city. This was done so following an apparent but unknown deal with the Metal Head leader, Kor. Krew sent Sig to do this, which apparently was also to serve as a suicide mission, before Jak and Daxter came to his aid after killing Krew in the missions "Find Sig in Underport" and "Escort Sig in Underport". After opening the underground tunnels, Sig gave the key to Jak, but it was not seen again.
The ruby key is a large device crafted of Precursor metal, with a ruby gem implanted in its octagonal handle, or bow. It then has a straight shaft, or blade, with smaller other channels that are used similarly in the form of a normal key. Despite its rather larger size for a key, it appears to be seemingly lightweight.