Kyber crystals have been part of the Star Wars canon for years, mainly in Star Wars: The Clone Wars and Star Wars Rebels. However, Rogue One: A Star Wars Story was the first time they appeared on the big screen. Before now, the crystals were best known to be a lightsaber's source of power, but in Rogue One, the Empire was using them to power the Death Star's superlaser. The main kyber crystal seen in the movie was the one around Jyn Erso's neck that her mother gave to her, but it turns out that there was also one hidden inside Chirrut Îmwe's staff.
In Star Wars Rogue One: The Ultimate Visual Guide (via ComicBook.com), it's mentioned that a kyber crystal is located within the metal component of Chirrut's staff in what Lucasfilm Story Group executive Pablo Hidalgo called a "Crystal containment lamp." However, it's worth clarifying that rather than being a full crystal, the staff's artifact is described as a "sliver." While this still doesn't mean that Chirrut (played by Donnie Yen) was a Jedi or Force sensitive, there was a benefit to him having that kyber sliver.
Apparently kyber crystals can create a harmonic sound only Jedi can hear, but thanks to Chirrut's own enhanced hearing due to his blindness, he was able to pick up on that sound. This might also be the reason why he knew that Jyn had her own crystal. As the book states:
Designed as a symbolic source of inner illumination, it also allows Chirrut to better gauge where the end of the staff is, as he can hear both the battery and crystal harmonic.
While one of Rogue One's earlier drafts did contain some Jedi, the final version lacked any Force-manipulating individuals other than Darth Vader. Chirrut Îmwe, however, served as the Force's main representatives. When Rogue One begins, one of the last Guardians of the Whills, the protectors of the Temple of the Whills, which contained Jedha supply of kyber crystals in the Holy City until the Empire looted it. While Chirrut kept this faith after he and his brethren were tossed out onto the street, his friend, Baze Malbus, lost his spirituality.
Even though he couldn't control the Force the same way the Jedi could, Chirrut Îmwe proved himself to be a powerful warrior in Rogue One. Along with honing his other senses, we now know that the kyber sliver in his staff also proved to be helpful when taking down stormtroopers. It just goes to show how many uses the kyber crystals have, and who knows, maybe new secrets about them will be revealed in future movies.
You can see Chirrut Îmwe and his Rebellion allies in action now, as Rogue One: A Star Wars Story is currently playing in theaters.