In Greek mythology, what was Medusa's "hair" made of?

Medusa is best known for having hair made of snakes and for her ability to turn anyone she looked at to stone, literally to petrify. Multiple works by ancient sources, such as Homer, the eighth-century B.C. poet Hesiod, and the fifth-century B.C. lyric poet Pindar, provide a wide-ranging and diverse picture of the fabled creature. According to Hesiod’s Theogony, she was one of three Gorgon sisters born to Keto and Phorkys, primordial sea gods; Medusa was mortal, while the others, Stheno and Euryale, were immortal.

Source: MET Museum