You might say it's just too small to be traditionally weaponised, but big enough for a personality and singing? Galen has a few small crystals as well, no bigger than the little souvenir one Jyn wears in canon, but he's going to put them in an array together to work his little balcony irrigation system -- a civilian energy use miniature proof-of-concept for the intrinsic peacefulness of his work. A scientist's reaction to his science going catastrophically wrong or being catastrophically misused is making more science, after all.