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:
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.