The exact nature of what a Soul Crystal actually is in the world of Final Fantasy XIV is mostly unexplored. We know they carry a large amount of knowledge related to the job they were created for within them and have the deeds of previous wielders carved into them (with the exception of newly-created jobs such as Machinist).
I’ve long wondered exactly how they work in-universe, and in the absence of anything official, I’ve concocted the following headcanon regarding them.
Soul Crystals are semi-sentient entities
I imagine Soul Crystals have to work a lot like Jedi and Sith Holocrons, at least in the old expanded universe. They have to be able to sense the potential and current skill in those who touch them, the better to know where to begin instructing them. However, where a Holocron could conceivably do this with an artificial intelligence, a Soul Crystal does not have that possibility. They are described as having a large quantity of aether within them, as well as vast amounts of knowledge including the history of their previous bearers. That sheer amount of magic and energy has to have granted them a similar state of semi sentience, not able to truly think and comprehend, but more of an instinctual behavior that allows them to evaluate and dispense knowledge within them as appropriate. They aren’t capable of direct communication beyond placing knowledge, feelings, and instincts in their bearer’s minds.
For the first 40 levels in which you bear a soul crystal, it is primarily your direct teacher
WIth some exceptions, your job trainers aren’t usually the ones providing you instruction in the abilities you gain. After completing job quests, you’ll often see a brief cutscene with a caption stating that some form of a memory has awoken within you after you completed whatever trial you’d just accomplished. Other times you’ll gain an ability at a level where there IS no job quest.
That’s the Soul Crystal determining that you’ve sufficiently advanced enough in everything you’ve previously learned, and that you’ve passed an appropriately challenging trial to show you’re worthy of the next skill or spell it has to teach you, and it thus directly imbues the knowledge in your brain in whatever form is appropriate for the job. Some might grant you memories of previous bearers learning the skill, others might show you the formulae for the specific spell, it all depends on the job
From the 60 to 70 level range, the Soul Crystal is letting you take the lead, while still guiding you when needed
From 60 to 70, you’re not getting abilities or traits after quests anymore, you’re getting them when you hit the respective levels, at least until 70. At this point, the crystal is letting you take the lead on figuring out new abilities for yourself. It’ll give you a basic idea and from there, you take over trying to figure out what it’s trying to teach you. It’ll guide you along the way with things like subtle corrections of form, but otherwise it wants to see if you can arrive at the conclusion yourself, and when you do, it provides reinforcing thoughts and releases the full amount of information to you. This all culminates at level 70, when it’s looking for a trial to arise that proves you’re worthy of the culmination of its teachings.
From 70 to 80, you’re no longer the student. And the Soul Crystal isn’t teaching…it’s recording what you create.
Everything you gain from leveling from 70 to 80…is your creation. The Crystal has nothing left to teach you. You’ve achieved mastery over the Job as far as anyone before you has gone…and so now the crystal is recording YOUR advancements in the arts. New spells, new skills, new modifications to existing abilities, you are the one who will provide these to the next generation of your job. The Crystal has begun etching your deeds into its facets, and absorbs your new knowledge into the aether within itself.
It would be very proud of you if it was capable of such a thing.