The Ruby Key is an important Precursor Artifact in Jak II.
Mar had created the Ruby Key during his rule. The key itself unlocks some ancient doors in the Underport. Krew had sent Sig down there to do so, as he wanted to betray Haven City for an unknown offer by Metal Kor. When Sig opened the doors Metal Heads started streaming in.
Jak and Daxter had to retrieve the Key in a mission for Krew, where they had to enter the Sewers and drain the water. After doing this, they found Mar's Statue, which Krew had smuggled down there years earlier, during his "younger days". Daxter climbed up the Statue and, with some effort, pulled the key out from Mar's stone grip.