Auracite, also called Holy Stone or Zodiac Stone, is a form of magicite. They are crystals, and are magical in some way and sometimes aligned with the zodiac signs.
Auracite is mentioned by Unukalhai as being involved in the fate of another world. It was used by its realm's champions to contain the power of primals, but due to a fatal flaw, it gradually bled out the energies and changed its wielders into fiends of endless appetite, devouring their world's life force.
A similar object, the White Auracite, was a pale stone that could be used to trap an Ascian, allowing them to be destroyed by striking it with sufficient amounts of aether. The Warrior of Light uses these to destroy Nabriales, Igeyorhm, and attempt to destroy Emet-Selch, and Elidibus.
A form of auracite created by Ultima, an otherworldy being, can be found in Dalmasca. This type of auracite was also used centuries ago by the Zodiac Braves. Auracite can transform their wielders into Lucavi, by feeding on and amplifying their greatest desire, and then absorbing their aether and transforming it before returning it to their body, turning them into monsters. The form the auracite's wielder takes is dependent on the individual, not on the auracite; although Argath Thadalfus and Ba'Gamnan both used the Taurus auracite, they became entirely different Lucavi: Duma and Yiazmat, respectively.
Auracite can also be used for good - the Zodiac Braves turned themselves into heroic Lucavi to keep Ultima sealed and test her would-be vanquishers. Even after Ultima is gone her auracite still works: Sadr rem Albeleo and Gilbrisbert quo Buteo have stones that can transform them into Adrammelech and Cúchulainn, respectively, and the sword Save the Queen was crafted from auracite by the Allagan Empire.
Cid Garlond believes that the Heart of Sabik encased in the Ultima Weapon was a type of auracite, and in the Japanese version Lahabrea explicitly names it the Black Auracite Sabik. Synthetic auracite based on the Heart of Sabik is employed by the VIIth Imperial Legion as cores for their Ultima Weapon-based prototypes: the Ruby, Sapphire, Emerald, and Diamond Weapons. They contain Oversouls based on skilled Garlean military personnel (Nael van Darnus, Regula van Hydrus, Gaius van Baelsar, and Alfonse, respectively), fusing the pilots with the Weapon's core and replacing their personalities with another soldier. A flaw in the Ruby Weapon's Oversoul turned its pilot into a manifestation of van Darnus, though this was rectified with the Sapphire Weapon.
The story of the Zodiac Braves is a legend regarded by the Church of Glabados as the central doctrine of its teachings. The twelve Zodiac Braves are said to have fought the Lucavi, driving them back into the spirit world. The legend foretells that the Braves, carrying the Zodiac Stones, would come to save humanity should they ever be in peril.