The Golden Crystal

During the Dream arc, when Helios finally reveals himself to Mamoru and Usagi, he explains to Mamoru his role in the past, and the power Mamoru possesses. From an Alex Glover translation:

You protected this planet Earth. The Silver Millennium from without, and the Golden Kingdom from within. And now that mission is carried in the two crystals of your kingdoms. The Phantom Silver Crystal is the holy stone of the Princess of the Silver Millennium. The Golden Crystal is the holy stone of the Prince of the Golden Kingdom. They send out your power.

It is at that point that Mamoru understands that he is the carrier of the Golden Crystal, which is much akin to the ginzuishou. This is very important as it shows that Mamoru's role as guardian of Earth is juxtaposed with Usagi's mission as guardian of the moon and Earth. By this, I am also led to believe that the two crystals are a symbol of their destiny to be together and to fight together.

The Golden Kingdom & Elysion

It is revealed in the Dream arc that Prince Endymion resided in the Golden Kingdom. Along with the Shittenou, he also had a guardian preist by the name of Helios. They never met during that time, for Helios prayed for the planet's safety within the seclusion of his shrine in Elysion.

Not much is known about the Golden Kingdom. We do know, however, that Elysion is the land of Earth's dreams and plays a large part in connection with Mamoru. In the manga, Elysion was also guarded by two maenads, who acted as preistesses. Helios informs Mamoru that the four souls are all connected to each other - Mamoru, Helios, Earth and Elysion - and when on falters, so do the other three.

Usa-ko; the Tsukino Usagi/Sailormoon Fanlisting La Smoking Bomber; the Chiba Mamoru/Tuxedo Kamen Fanlisting Forever I Promise; the Usagi & Mamoru Fanlisting