No point in ruining a good chisel
I'd just use another hard rock and gently tap it against the sharper corners of those rocks to make it less pronounced and to have the outcome be a bit more random. Should be able to gauge fairly quickly how hard of a 'tap' you'll need to give it. So long as the edges aren't so uniform, I think the flatness of the side will be less noticeable. Can always try on one of the smaller pieces and see how it goes.