This is the explanation for Flurry Hearts crystal situation.

When I recall the crystalling episode, I mostly remember the scene in which Spike told the old tradition of Crystalling. Every foal or filly born under the Crystal Empire has to have a ceremony where they are blessed with the power of the Crystal Heart. It was the only point where we got a glimpse of the Crystal empires older forgotten history so it was interesting. Then the Crystal Heart broke and with the help of the princesses, Sunburst, and Flurry Hearts magic contained in the crystal shard they managed to restore the Crystal Heart. End of episode.

That tradition was supposed to be followed to the same extent, and it wasn't. Now the part as to why Flurry Heart in my universe grows crystals on her skin is simply because her magic was fused into the newly created Crystal Heart at a very young age. With the y know added bonus of multiple alicorns and I assumed an entire frickking city of ponies adding their power to make sure the Heart is reformed, that is going to bring a lot of magic power and repercussions. That being that she has formed a direct connection to the Crystal Heart.

In other words, a living, walking, Crystal Heart.

She was only five years old when it happened to appear itself, on a night when the crystal empire had the worst of its snowstorms during the winter. Flurry Heart went outside the palace to explore, not knowing that a storm was soon going to happen. Once the storm started, she was lost, and later stumbled upon a small foal also caught in the terrible weather. The foal was too cold to move and so was she, thus she used her feathers to make sure he was warm and her magic to make sure the snow wasnt burying them alive, but her magic was unpredictable and at that moment she couldnt make a shield for both of them much less herself, but somehow she managed to get some magic to work, though it was found later that it wasnt her magic at all. It was hours later when help finally arrived and both were promptly taken to safety.

Unfortunately despite her sacrifice, the foal died of hypothermia. Thus she vowed to never let another pony die due to her inability to control her magic well. When she got older she continued to practice her magic, the crystal does grow during her training, but as long as it doesnt get too far they can fall off after a few hours. Shes glad that the Crystal Heart can help her control her unpredictable magic. The whole idea of sacrificing herself for the benefit of her people is deeply embedded in her. She believes- even if it scares her- that its the most honorable way for her to go if it came to it.

I said before that Flurry has limited power; the stronger the spell she uses to protect the more the quartz grows. The reason why is because the only thing that can protect the empire is the Crystal Heart itself, which means in order for Flurry to fully protect her home and subjects she needs to become the very thing that protects said empire.

Flurry and her family are, of course, aware of her situation, it is not reversible unless you destroy the Crystal Heart (again) and leave Flurry completely powerless. Its hard trying to convince her to change her mind about being selfless she absolutely dislikes it when somepony tries to change her mind.

How far of magic is it too far? Well defeating Sombra is dangerously close to having at least two limbs crystalized and half a face. Anything bigger than that, lets say, a fully hydrated Tirek scaled, is certainly a big No. Flurry is aware of her connection to the Crystal Heart that bond allows her to feel the strings of the city all buzzing with life and occasional odd whispers coming from the heart itself. As for taking counted measures, she gets trained for more simple spells

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My bad for the late post, school and all (eOe)

Thats all! I certainly wanted Flurry to have a much harder life other than Princess has princess duties to live up to even if it's a little different. ( I didn't like how she was born with such strong powers, at first.) Also, I like the idea of the crystal heart using Flurry as a host for ensuring the Crystal Empire is safe. Cause why not.