While we’ll get to their involvement in Rogue One in a bit, it’s best to start by talking about the most significant way Kyber Crystals have been present in the Star Wars universe.
This is a Force-guided process, where the younglings are drawn to their own specific crystals.
When building their own weapons, the force-sensitive children are sent to the Crystal Caves of the planet Ilum where they are to harvest crystals that they will use to construct their own lightsabers.
First examined in the Clone Wars Season 5 episode “The Gathering,” and first identified by name in the episode “A Test Of Strength,” Kyber Crystals are not just the key component in a lightsaber, but are also an important part of an step in a youngling’s path to become a Jedi.