Life Crystals were the subject of my biggest concern when creating the world of the Misplaced Hero. The thing I thought most likely to break suspension of disbelief. The thing I thought most likely to cause readers to quit reading.
They were also the thing that took the longest to nail down. Who saw them, what properties did they contain, how did you do a stealth run with a glowing light floating over your head?
Clearly a video game aspect, I initially thought that everybody might have one. That didn't last long. That would get too busy and difficult to manage pretty quickly.
My next thought was that only the gifted would have them, but everybody could see them, or their lack. Therefore, every ungifted as well as gifted would be able to easily see whether someone was gifted or not. That idea lasted several iterations, going away and coming back in one form or another as the world grew.
In the end, because of story concerns, I felt it necessary to limit the ability to see the things to the gifted alone. One of the aspects of the gift, you might say. More on that later.
What about animals and/or monsters? On Mund, the line sometimes blurs. The dire hares, for instance, are a mutation of mundane hares. Would they have life crystals? Would the dire wolves have them?
It wasn't a thing I worried about overmuch in the early stages of the book, since, whether they did or didn’t, it made no difference to Jack. He couldn't see them anyway. It was Luciandro that helped me out with his revelation regarding the gift.
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If everything and everybody has a gift, and the main difference is the strength, then everything would have a life crystal. The big difference between 'gifted' and 'ungifted' would be how brightly the crystal manifested. Mundane hares would have crystals, but they'd be so weak that only very high ranked detection spells or gear would be able to see them. Dire hares would have crystals significantly brighter, and visible to the naked eye of any gifted who looked. And so on.
The reason neither Rosaluna nor Mohrdrand, at their elevated ranks see the crystals of the 'ungifted' is that very high ranked in this instance translates into 'higher than that.' Millie, with the strength of her gift, might appear to Rosaluna, for instance, to have a shimmer above her head, no more, and that only if Rosaluna took it into her head to look for one, and why would she do that?
I dealt with the stealth aspect by borrowing a page from Games Workshop's playbook. Specifically their treatment of the 'shoot me pole' that many love putting on the backpacks of the space marines. If all you can see is the pole, you can't see nothin'!
I dealt with this by finally realizing (in my own head) that the crystal wasn't necessarily a physical thing. It was a manifestation of the physicality of the creature it marked. A flag, no more. If you could see the creature, your gift would translate its properties and display a marker in the form of a life crystal.
The belief of the Mundians that it was the crystal itself that the window imprinted on was incorrect. Folklore grown from the history showing that nobody who didn't manifest a crystal had ever been recognized to have a useful gift. The window only requires blood and physical contact, as we now know, given Jack's success.
Further, the stronger the gift, and the brighter the crystal, the further away it can be seen, as the strength of the magical aura the body exudes manifests more clearly.
That's where we are currently. The crystals may or may not be further refined going forward, but I'm pretty confident that no future changes will contradict anything currently set down as canon.