Obsidian

Obsidian was a hard, black variation of glass, found in areas of volcanic activity. It was used for making blades on the planet Karagula by the surface-dwelling Karagulans because of its high quality. (PROSE: The Colour of Darkness)

Gabby Gonzalez read a legend claiming that Aztec magicians used mirrors of black obsidian to deflect bad spirits. The Tenth Doctor claimed that Tezcatlipoca liked black obsidian. (COMIC: Revolutions of Terror)