The shell is coiled , calcareous , mother-of-pearl-lined and pressure resistant . It will implode at a depth of about 800 m. The nautilus shell is made of 2 layers : the outer layer is a matte white , while the inner layer is a striking white with iridescence . The shell is coiled , aragonitic , nacreous and pressure resistant , imploding at a depth of about 800 m. The nautilus shell is composed of 2 layers : a matte white outer layer , and a striking white iridescent inner layer .