The Golden Crystal (ゴールデン・クリスタル) appeared in both the manga and anime, but had different uses and origins in different continuities.
In order to break the seal of the Golden Crystal, Helios searched for the "chosen maiden," a warrior princess possessing the power of the Silver Crystal. This maiden was later revealed to be Sailor Moon. In Helios' words, only the joined power of the Silver Crystal and the Golden Crystal could defeat the Dead Moon and save the planet.
In Act 42 of the manga, when Sailor Moon's Silver Crystal became the more evolved Silver Moon Crystal, the seal of the Golden Crystal was broken. With its power and the powers of the Solar System Senshi's Sailor Crystals, Sailor Moon transformed into Eternal Sailor Moon and defeated Nehellenia, sending her back into her mirror. Helios then took the Golden Crystal and fixed it at the top of King Endymion's royal staff, triggering Tuxedo Mask's and Sailor Moon's transformations into King Endymion and Neo-Queen Serenity. Once the moon was freed from Nehellenia's curse, the King and Queen drew the darkness out of Nehellenia's mirror and exposed it to the moonlight, destroying the power of the Dead Moon forever.
The appearance of the Golden Crystal in the manga was similar to that of Sailor Moon's Silver Moon Crystal: a crystalline lotus with a shining ball at its center. When Sailor Galaxia destroyed Mamoru's body in Act 44 and took his Sailor Crystal, the Golden Crystal was seen in the lotus form, then its core opened and revealed a hexagonal crystal similar in appearance to the anime Golden Crystal.
In the anime, the Golden Crystal was a crystal guarded by Helios , the priest of Elysion, and was said to have enough power to destroy entire planets.
Since the power of the Golden Crystal came from the beautiful dreams of people, and the Dead Moon was created from the wasted dreams of those who had forgotten how to dream, the Golden Crystal was the only way to defeat Queen Nehellenia and her underlings.
In the past, the Golden Crystal was kept in the sanctuary of Elysion. Helios stated that the Crystal chose its user, but only those with beautiful dreams could be users of the Crystal. To prevent an evil being from taking a hold of it, the Crystal would envelop itself in a golden flame. In order to be able to use the power of the Golden Crystal, Nehellenia captured the Golden Mirror of dreams where Helios had been hiding in the form of a pegasus. Nehellenia then used the Golden Crystal as an amplification device to increase her evil powers. This way she could easily break the seal of the mirror in which she was sealed and defeat the Sailor Senshi.
When Sailor Moon and Sailor Chibi Moon took a hold of the Golden Crystal, they joined their powers with the power of the dreaming people on Earth to destroy the Dead Moon Circus and seal Queen Nehellenia once again in the darkness of the new moon. The Crystal was then returned to Helios, who took it back at Elysion.
In the anime, Mamoru had a golden Star Seed that made him representative of the planet Earth, but it was not the same object as the Golden Crystal.