The Great Crystal is a large crystal, and one of the focal points of the plot of the Final Fantasy Crystal Chronicles series. It is kept glowing by the memories of the people and has the power of altering the fates of every single person in the world. Originally it was part of a whole piece and inside it was the great crystal's memory, a sub-universe where anything is possible.
Final Fantasy Crystal Chronicles
The Great Crystal is no longer in one piece; at some point between the end of Ring of Fates and the start of Crystal Chronicles, a meteor, containing the Meteor Parasite, fell from the sky and shattered the Great Crystal. The meteor began emitting Miasma, which spread throughout the world.
The survivors discovered the shards of the Great Crystal kept the miasma at bay and built settlements around the scattered shards, which were the only thing that could protect them. The effect did not last and myrrh was needed to replenish the protective effect and thus began what became known as the Crystal Cycle.
Final Fantasy Crystal Chronicles: Ring of Fates
In the Golden Age, the Great Crystal blessed the planet by giving them clean air and purifying the water and repelling most of the malefic, tempting, and consuming power of the crimson moon. Certain pure-hearted individuals can tap in to the energies of the Great Crystal, called Starsingers.
After the fight with the Lich, Yuri, Chelinka, Alhanalem, Gnash, and Meeth are transported from the Shadowland in to the Great Crystal's chamber. The crystal cavern is surprisingly large, and the crystal on the surface in the city of Rebena Te Ra is "just the tip of the iceberg". After Lich's final defeat King Kolka reveals to the party the origin of their planet and the repelling forces of the Crystals and the Moon.
After the storming of the defiled Crystal Temple, and after Galdes alters reality, the Great Crystal still existed, and Galdes was going to kill Yuri and Chelinka if they defied him, and use the Lunite power to turn the Great Crystal crimson.
The Latin phrase Ibi in Crystallum inscibuntur omnia fata that appears on the artwork depicting the protagonists of Ring of Fates is a straightforward description of the power crystal: it means literally "There in the Crystal all fates are written". The crystal records the events of the past, present, and future and when one uses the power magic, in the power of the Starsinger or Magicite, they choose a parallel universe, diverging from the world where they normally would have taken physical action. The reality they choose is, usually, hardly different than of the alternate choice, but in the immense magic power that Galdes, Yuri and Chelinka used, this altered reality, although the former used the power of the moon.
When Galdes is defeated in his final form, Yuri appears in the Great Crystal's memory. With the power both he and his sister have, they can tap into the more powerful source: the Great Crystal itself. With it they can do absolutely anything they can imagine. With this power the twins make an alteration on the entire universe: their parents, Aleria and Latov, alive, Galdes gone, and peace among the entire world. In an interesting turn of events, the twins chose to change the Crystal's nature: in every parallel universe the Crystals were an omniversal anchor.
No matter the alternate universe, the crystal would stay the same, and those whose blood was infused with it. King Kolka was one of those few individuals, in every universe his wife and daughter would still be dead. But when Yuri and Chelinka altered the crystal and omniverse they decided to revive Princess Tilika in every universe.
Final Fantasy Crystal Chronicles: My Life as a King
In My Life as a King, a piece of the Great Crystal appears within the center of Padarak. It grants King Leo the power of Architek to rebuild his Kingdom that was destroyed during the time when miasma covered the world. It is later discovered, the former ruler of Padarak, the Dark Lord's, heart resides within the crystal after his body had been destroyed once the miasma cleared.
When he appears before King Leo, the crystal glows an eerie dark purple. After enough Elementites is collected to restore his body, the Dark Lord leaves the crystal and over time it glows brighter as the Kingdom grows larger. It is never made clear if it was the crystal speaking to Leo or the Dark Lord trapped within it. Its motives are unclear as it had originally granted the Dark Lord the power of Architek, although this could have been before the miasma had turned the Dark Lord into a monster as it is said that he was once human.