Quote Originally Posted by HalfTangible View Post
Mix it with other material to imbue it with the slateshell's anti-magic properties?
Would powdered slateshell mixed with steel(or other metals) still retain its protective qualities?

Build prisons for mages?
Depends a lot on how magic works in this setting - though not being able to just blast through the walls is probably a start.

Magic resistance alone makes it pretty useful as a material.