I use a mallet to break up my larger rocks into smaller ones and then use a piece of the rocks itself to chisel away at them a bit to reduce sharp ages and just fine tune in general. Some rocks take better to this than others. The ones you have may be prone to shattering so I'd exercise a lot of caution with safety gear where appropriate if you do try it. Good luck.
I have a feeling you're right about the shattering potential. They may take to some light work with a chisel... Thanks, Zlookup.