The Soul of the Paladin.
A 
Soul Crystal is a piece of equipment in Final Fantasy XIV that changes the character into the job related to the soul crystal.
Background[]
What a soul crystal actually is is largely unexplored, but its stated purpose varies from job to job and it is consistently described as containing a wealth of knowledge related to the job within it, being an extreme concentration of aether, and being carved with the deeds and exploits of past people who held the job's mantle. The exceptions to the latter rule are 
Machinist and 
Blue Mage, jobs new to the world with soul crystals described as pristine as a result. As demonstrated in the case of the new machinist soul crystals, soul crystals can be manufactured.
Generally a magic-related job attributes some importance of the crystal to their spellcasting, such as the crystals for 
Black Mages being necessary to prevent self-immolation from loss of control over spells. The crystals for 
Summoners are used as a catalyst for the summoning of a primal's Egi, and 
Scholars' crystals are used to store their fairies when they are not summoned so that they are not dismantled and can continue to learn. Physically-related jobs, such as 
Ninjas, may describe certain techniques as being extremely difficult or impossible without their corresponding soul crystal, though some jobs will not offer an explanation for their soul crystal whatsoever. 
Dark Knights function without a soul crystal, as the Warrior of Light's at one point shattered and needed to be repaired through absorption of the crystal's aether, but its state of disrepair had no adverse effect on the Warrior's techniques or capacity for them.
Some soul crystals may also contain the literal souls or consciousness of previous bearers, such as the Warrior of Light's soul crystal for 
White Mage containing the soul of A-Towa-Cant. Additionally, a soul crystal may contain a physical item, such as the soul crystal for 
Dragoon containing the chest-piece for the Drachen set.
Gameplay[]
Soul crystals are equipped as 
Accessories. Equipping a soul crystal on one of the combat classes will change the class into the associated job, such as a 
Gladiator becoming a 
Paladin when equipped with the Soul of the Paladin. Jobs have greater base attributes than their base classes, and several abilities can only be accessed as a job. Formerly, it limited the amount of cross-class actions a player could use, and some cross-class skills were available only to certain base classes and not their jobs, giving base classes some niche over their jobs, but with cross-class skills being replaced by role actions in Stormblood, these restrictions were removed. A soul crystal is also a requirement to equip certain weapons, typically the various stages of weapon from each expansion's ultimate weapon quest chain, as well as Scholar weapons that are not designed for use with 
Arcanist. New jobs introduced as of Heavensward lack base classes, and their soul crystals are equipped by default and cannot be unequipped.
Soul crystals for disciples of the hand function differently from those used by disciples of war or magic. They do not change them into a job, instead providing some non-scaling bonus attributes and granting them access to the Careful Observation skill. Only three disciple of the hand soul crystals may be owned at any given time. Characters with three such soul crystals must exchange them for new ones if they wish to change their specialization.
Etymology[]
The soul, in many religious, philosophical, psychological, and mythological traditions, is the incorporeal and, in many conceptions, immortal essence of a person, living thing, or object.