The Occuria created auracite from two large crystals called auraliths that contained tremendous amounts of magick, in much the same way they had created nethicite from the Sun-Cryst. The Occuria gave this auracite to Feolthanos, the leader of the aegyl, intending to use him as they had used Raithwall. But Feolthanos enraged the Occuria by refusing to serve them; instead he and the aegyl fled to Lemurés, where he used the auraliths to create a barrier around his floating world.
By the time of Final Fantasy XII: Revenant Wings, aegyl regularly use pieces of auracite to summon Yarhi (called Espers by those from other parts of Ivalice) through a device called the Ring of Pacts. However, the auracite has another, darker effect the aegyl are unaware of: creating a guardian Yarhi strips away part of the user's anima and transfers it to Feolthanos. Thus through thousands of years using auracite to defend themselves against unbound Yarhi, the aegyl have been reduced to little more than soulless shells while Feolthanos has grown more powerful. Interestingly, while the party speculates that the sky pirates who have begun using auracite may have started to lose their anima, none of the party members from outside Lemurés seem to be affected.
The auracite in this game does not help in summoning Espers. Instead, they hold the spirits of Lucavi, some of which appear as Espers in Final Fantasy XII and are the new name of the twelve Zodiac Stones of the Zodiac Braves and are crystals of auracite instead of Holy Stones. They may not be entirely evil, as one helped revive Marach after he was shot while defending his sister. Marach claimed to have heard a calm, gentle voice telling him to go back. Ramza has said that perhaps the stones are raw power, and that the will of its wielder shapes it. To release the Lucavi seal in the auracite, the stone needs a vessel. It is believed that this host must have the same Zodiac sign of the auracite that the vessel holds and requires that person to become the Lucavi's vessel, but this is not necessarily true, especially with the case of Wiegraf Folles (who is a Virgo, but is possessed by Belias, an Aries). In the case of the Lucavi leader, Ultima, there can only be one vessel for her auracite in so many generations.
Added by Icysugarspike
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Vaan holding an Auracite in Final Fantasy XII: Revenant Wings.
Artwork of an Auralith in Final Fantasy XII: Revenant Wings.
Aries Stone in Final Fantasy Tactics.
Cancer Stone Final Fantasy Tactics.
Capricorn Stone Final Fantasy Tactics.
Gemini Stone Final Fantasy Tactics.
Leo Stone Final Fantasy Tactics.
Libra Stone Final Fantasy Tactics.
Pisces Stone Final Fantasy Tactics.
Sagittarius Stone Final Fantasy Tactics.
Scorpio Stone Final Fantasy Tactics.
Serpentarius Stone Final Fantasy Tactics.
Taurus Stone Final Fantasy Tactics.
Virgo Stone Final Fantasy Tactics.
