Superman the movie, Jor-el head turning to crystal after Kal-el returns to Fortress of Solitude
original movie prop
Here is the clearer crystal mask used in the Fortress scene when Kal-el returns after his long ten year journey being taught by Jor-el. Upon returning Jor-el speaks to Kel-el dressed as Superman and as Jor-el finishes his head slowly turns to crystal. In the video it's at the 3:54 mark.