The "Golden Crystal" is a supremely powerful crystalline artifact that was a major driving plot point in both Sailor Moon SuperS and the Dream saga of the manga, though both their origins severely differ between the anime and manga.
It is the Earth's equivalent of the Silver Crystal and equals it in strength and power.
Manga and Crystal/Eternal
In the manga, the Golden Crystal was the Sailor Crystal of Mamoru Chiba. It is what gives him his power and allows him to transform into Tuxedo Mask, although Mamoru was unaware of its existence. When he was Prince Endymion he used it to protect the Golden Kingdom of Elysion.
It worked very similarly to the Silver Crystal and actually added tremendously to the Silver Crystal's own supreme power. When in the cursed land of Elysion, Super Sailor Moon realized that all the times Tuxedo Mask would be there to give her new strength and bravery was because of the Golden Crystal inside of him. The Golden Crystal was sealed within Mamoru and according to Helios, Eternal Sailor Moon was the only one who could break its seal and allow its full power to be used. When activated, the Golden Crystal would work together with the Silver Crystal to revive Elysion and transform Mamoru and Usagi into King Endymion and Usagi into Neo-Queen Serenity; after which it was affixed to King Endymion's mystic staff.
In the 90s anime, there was no indication that the Golden Crystal had any connection to Star Seeds. The Golden Crystal is shaped like a diamond and was placed on the head of Helios/Pegasus; the Guardian and high-priest of Elysion. After Queen Nehelenia attacks Elysion, Helios (as Pegasus) escapes from his physical body to prevent Queen Nehelenia from obtaining the Golden Crystal's great powers for evil ends. After Super Sailor Chibi Moon is captured, he is reunited with his physical human body, allowing Nehelenia to take the Golden Crystal from him intact.
With her newfound strength and freedom, she uses the immensely strong, dream-infused abilities of the Golden Crystal against the five Super Sailor Senshi. The Amazoness Quartet steals the Crystal from the Queen and gives it to Super Sailor Moon. She and Super Sailor Chibi Moon combine their power with the power of dreams from the people of Earth and it is used to seal Nehelenia into the new moon. An unconscious Super Sailor Chibi-Moon was seen holding it in her left hand. Helios then takes the Golden Crystal back to Elysion as its guardian once again.
Originating from the golden kingdom of Elysion, the Golden Crystal derives all of its great powers from the lovely dreams of the humans on Earth. The more people who fail to achieve their sweet dreams, the weaker and weaker the Crystal will become until it is powerless. It is only fully energized and utilized at its strongest when everyone's good hearts come together and continue to dream.
The Crystal emits dazzlingly bright rays of golden-yellow light that can greatly increase the powers of others; such as when it bestowed the Moon Kaleido Scope upon Super Sailor Moon and the Crystal Carillon on Super Sailor Chibi Moon. Another ability it possesses is maintaining the current forms of human-like creatures; as it had done so for the Amazon Trio.
It also makes its wielder practically invulnerable to even the strongest of attacks. Also, the Golden Crystal seems to be a sentient and autonomous artifact; as it has a mind and will of its very own; equipped with its own emotions and feelings. It only allows one to utilize its full powers if one has a good heart full of lovely dreams.
- In Episode 197, Mamoru's Star Seed bore a resemblance to it even though the Golden Crystal had returned to Helios. This is an implication that Mamoru is indeed more connected to the Golden Crystal than he realizes.
- In the video game, "Bishoujo Senshi Sailor Moon: Another Story", it was replaced with the Rose Crystal.
- In the Cloverway English dub, it was known as the Golden Crystal of Dreams.