Lyra's Kyber necklace was originally whole, but broke when she took it with her. The first half reading "Trust in the" was given to Galen, who did not believe in the Force and told Lyra she was superstitious. He secretly kept the crystal and brought it with him everywhere.

Lyra, who already had trust and did not need to be reminded kept the "Force" half of the crystal and made it into the necklace that she wore constantly until the day the Homestead was attacked and she no longer needed the protection.

Knowing the risk she was to take and that she'd likely not return, Lyra passes the crystal to her daughter who unknowingly forms a connection with the Force. She uses this connection to "magically" survive even the most unlikely of situations throughout her life.

Since the crystal was passed on while Galen was already heading out to meet with Orson, he did not know that Lyra no longer had it and he assumed that it was buried with her on Lah'Mu... but he could still somehow sense her spirit in his strongest moments when he thinks of her.

What neither Galen or Jyn realize is that they are vaguely communicating through their bond in the force- the two halves of a broken crystal trying to mend an equally broken family. Each time they speak it's fragmented and they assume they are seeing glimpses of the past.

Since Jyn's crystal is cracked its Force signature is easier to sense, leading her to meeting Chirrut in that market on Jedha. He mentions that the strongest stars have hearts of Kyber, referring to a lot but a little all at once. And of course, to the Erso family.

Then on Eadu when Jyn realizes how much danger her father is in, she accidentally projects her fears and absolute NEED for him to survive onto Cassian, who is on the peak attempting to line up a perfect shot on the platform. This hardened rebel soldier that would do anything for

his cause is suddenly struck by doubt- something he's never let control him before. He had his chance, but the two halves of the Erso's crystals linked for a moment and when he looked at Galen, Cassian saw his daughter and couldn't take the shot