ljuštura

1. (of molluscs, turtles) shell
   Na plaži smo skupljali prazne ljušture školjki. — We collected empty seashells on the beach.
   Kornjača se sakrila u svoju ljušturu. — The turtle hid in its shell.
2. (figuratively) shell; empty husk
   Posle požara od kuće je ostala samo ljuštura. — After the fire, only a shell of the house remained.
   Bez nje je bio samo prazna ljuštura. — Without her he was just an empty shell.

For eggshells and nutshells ljuska is more usual.