The is a gold crystal that is incredibly powerful, and is powered by people's dreams. It could enable the holder to rule the world. It takes someone of great power to possess the crystal. If people lose their dreams, the crystal loses its power, except in the hands of someone who possesses a kind of power to use it.
The golden crystal was in the possession of Helios. Queen Nehelenia wanted it so she could rule the Moon Kingdom, however she was unable to get it due to her evil intentions, so she imprisoned Helios, but was sealed away in a mirror. Later, Helios was imprisoned inside Nehelenia's mirror by the Dark Moon Circus, who wanted it so Nehelenia could both escape from her mirror and rule the world, but were unable to get it once again; Helios was separated from his body with Rini's dream light, taking the golden crystal with him as his spirit became Pegasus, entering Rini's dreams. Pegasus's horn was the golden crystal, and gave Sailor Moon the ability to use Moon Gorgeous Meditation.
The Amazon Quartet tried to take the golden crystal once Nehelenia had Rini trapped. Nehelenia eventually got the golden crystal, and was able to escape the mirror. Wielding the golden crystal, she was unstoppable against the Sailor Scouts until the Amazon Quartet (now on the side of good) switched it for a pineapple and gave it to Sailor Moon. Sailor Mini Moon (Rini) spoke to everyone on Earth through it so they would dream again, saying "Moon Cosmic Dream Action" to revitalize the crystal's power. Sailor Moon and Sailor Mini Moon then used it to shoot a gold beam at Nehelenia, defeating her and destroying most of the Dark Moon Circus. Though it was not enough to destroy Nehelenia, it eventually reduced her into an ugly old woman.
After the battle, Helios gets the Golden Crystal back