[@Skwint]

Oooh that's a good one. Something of that nature has never actually come up before, however I can see it making some good subplot points or twists.  My answer to that is it will dampen their abilities.

For the version where the kids have grown up knowing about their destiny... Loss or separation from their crystal will only dampen their control. They will no longer have the same level of control over elemental magic as they did before (regular shared magic as mentioned before will remain untouched). Power is determined by their own spirit. So in theory, they might still possess the same level of power... But can no longer control their magic. Which might cause them to mistakenly injure their comrades or themselves in the process.

For the version where they are from our world, and stumble across their destiny... That will be a dampening across the board. Since they are not from a world with a lot of magic, the stone pretty much fuel everything. They will retain some level of control over their abilities but the power will not be as great. The farther they are from the stone, the weaker they become. This will include their physical inheritances as well.