From what I’ve seen you can go with very thin sheets of shell or ground up shell you set into an epoxy. If you go the ground up shell route you can basically fill your engrave to depth.
If you want to cut from a solid sheet you’ll be looking at a lot more work. First, your engrave will not be uniform in depth. The laser deposits a uniform amount of energy into a non-uniform material. So you’ll need to sand your engrave area to remove char and teeny, tiny ridges. Note that the edges will be slightly shallower than the center - they don’t get as much time under the laser. Then you will probably have to sand the head down so the engrave is the correct depth. But, hey, fine instruments aren’t supposed to be easy? Right? As for cutting the shell here are two threads.