From what I've seen, Catalyst suggests it's not a kyber crystal. I haven't gotten to that point in the book though so I can't say for sure, and I'd be surprised if it wasn't a kyber.
If you haven't watched the red carpet premiere of Rogue One, there was a featurette that has not been seen anywhere else, featuring K-2SO. It does contain scenes from the movie that have not been seen before, so watch at your own risk.