Shells are pretty soft in the scheme of things, but can chip on the edges some times. You could probably cut them with a coping saw frame and a fine metal cutting blade. There may be special blades for this handsaw. The harder thing is to hold small pieces while you cut it.
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, and Longtime Forum Participant.