De/crystalization

the longer a crystalite is bonded to a human companion/keeper, the less crystaline and more flesh-and-blood it appears to become - so a crystalite bird eventually gets feathers, etc
the longer a human works with a certain element or color of crystalite, the more their skin/eye/hair colors change to reflect that. this can be reversed by stopping work with that crystalite, but the change back to “normal” human is very slow. most humans work with more than one kind of crystalite and will have multiple colors or patterns warring for space on their bodies.
there is a theory that if a crystalite is close enough to a human for long enough, it will become fully flesh-and-blood - in other words, mortal. this is a myth, as it is not thought that any single human could live long enough to help a crystalite progress this far. there is a matching myth, though, that a human could eventually become a crystalite and lose their mortality along with their warm-blooded body.
crystalites are frequently traded between friends, families, and total strangers - both as a form of secondary currency and as a way to help reduce the long-term impact of crystalite bonding on the human companion. most everyone gets crazy hair/eye/skin colors, sure, but when parts of your body start turning to patches of crystal, most humans tend to backpedal real fast.
of course, there are always fanatics. crystalite cyborgs, humans whose bodies have begun to become crystalite. they tend to have lifetime bonds with their crystalites, who also look the most mortal of any crystalites. humans don’t seem to be able to go past about ⅔ crystalization, while crystalites can become almost ¾ mortal flesh. this ratio is unaffected by volume or mass.
human children are largely unaffected by the process of crystalization and only seem to show hair color changes until after puberty has finished. if they bond with crystalites during childhood or puberty and keep them close in adulthood, those crystalites will have exceedingly slow and minimal crystalization effects on their human - a fact that some cultures exploit by only bonding with crystalites as younglings.
as far as humans can tell, a crystalite who doesn’t become too “mortal” by bonding with a human is effectively immortal. even a destroyed crystalite will eventually be reborn as essentially the same entity. some humans view decrystalization or mortalification of crystalites to be a negative thing and will frequently trade crystalites to avoid letting them become too much flesh-and-bone.
a crystalite who has bonded to a human and finds itself without a human (being traded, released, lost, etc) will slowly revert to its fully crystaline form, losing any hints of fur, feathers, or skin that it had gained. since this is such a slow process, however, many exchanged crystalites retain some of their mortal qualities by the time they bond with a new human, and their mortalification will continue from that point. (bonding with a human takes time, effort, and energy.)
a note: the crystalization process seems to be largely elementally-attuned. one human with three nature crystalites, who has each crystalite with her one at a time throughout the year, will still crystalize steadily. one human with an air crystalite, a water crystalite, and a wood crystalite, each with her one at a time throughout the year, will crystalize at less than a third of the speed of the first example - as the crystalization process is separate for each element, and when not in the presence of the bonded crystalite will revert even as another element’s crystalization advances. as a result, humans who enjoy having closely-bonded crystalites will often have seasonal companions who are dormant for the rest of the year.

Type

Physiological