What substance is the fictional superhero Superman not able to see through?

Superman is a fictional superhero appearing in American comic books published by DC Comics. The character was created by writer Jerry Siegel and artist Joe Shuster, high school students living in Cleveland, Ohio, in 1933. They sold Superman to Detective Comics, the future DC Comics, in 1938. Superman debuted in Action Comics #1 (cover-dated June 1938) and subsequently appeared in various radio serials, newspaper strips, television programs, films, and video games.

Although the element lead is the one substance that blocks kryptonite radiation (one of Superman's weaknesses), it is also the one substance through which Superman cannot see with his X-ray vision.

Superman has the ability to see through solid objects, usually with the exception of lead. Early stories assumed that hiding objects in lead would prevent him from finding them; however, more modern stories have Superman being able to take advantage of lead's x-ray opacity to do a wide scan of an area with his X-Ray vision in which the lead objects become immediately visible and then narrow his search to those specific locations.