Lore on Soul Crystals and their role for each job can be found here!
As for which jobs absolutely require a soul crystal to use, most of the disciples of magic fall into that category:
Black Mage, White Mage, Scholar, Summoner, and Astrologian absolutely require a crystal.
Machinist, Dark Knight, Warrior, and Dragoon heavily imply that a soul crystal is necessary to harness or control their respective powers.
The rest are more vague/subjective if the crystal is necessary.
As for which jobs absolutely require a soul crystal to use, most of the disciples of magic fall into that category:
Black Mage, White Mage, Scholar, Summoner, and Astrologian absolutely require a crystal.
Machinist, Dark Knight, Warrior, and Dragoon heavily imply that a soul crystal is necessary to harness or control their respective powers.
The rest are more vague/subjective if the crystal is necessary.
