LIT A cell is on the crust when at least one coordinate is 0 or 6. Counting them gives 218. The closed form n³ − (n−2)³ and the face-edge-corner form 6·5² + 12·5 + 8 give the same 218. The other 125 cells are interior.
AMBER Crust is a name for that counted set.
WALL The count does not say what the crust is made of.