The Heart of Mar is a Precursor artifact seen in Jak II. The Heart is a large ruby gem, carved into the shape of a heart and set in Precursor metal. The ruby lights up when activated. It is one of the items needed to activate the rift rider and serves as the start button for the vehicle. Once the rift rider was lost, when Jak, Daxter, Samos and Keira traveled through time, so was the Heart.
Supposedly Mar originally made the Heart and hid it in a statue of himself. It was stolen by Krew, but he lost it in a flood while smuggling it through the Haven sewers. Although later, Mar's statue was destroyed by Krew's men, Jinx, Mog, and Grim who blew it up and took the Heart in the "Escort men through sewers" mission. It was not until the "Get Heart of Mar in weapons lab" mission, and the follow-up mission, "Beat Krew in weapons lab", that Jak managed to take the Heart back from Krew.