In “Frosty the Snowman,” Frosty’s eyes are made of coal.
The detail comes from the lyrics of the Christmas song “Frosty the Snowman,” written by Walter “Jack” Rollins and Steve Nelson and published in 1950. The opening description presents Frosty with a corncob pipe, a button nose, and two coal eyes before explaining that a magical old silk hat brings him to life.
Coal was a traditional snowman decoration because its dark color stood out clearly against white snow, making practical eyes, buttons, and mouths. Carrots were also commonly used for noses, which is why Frosty’s button nose can sometimes be confused with the carrot nose associated with generic snowmen.
The 1969 animated television special keeps the familiar coal-eye design, but the song is the clearest source for the answer. Frosty’s eyes are not buttons, black stones, or pieces of charcoal in the canonical lyric: they are specifically described as being made out of coal.