Green’s Dictionary of Slang

a fortune teller’s crystal ball.

1953		F. Brown Madball (2019) 35: He pulled over the madball, the two-inch diameter crystal in its silver stand, and polished it lovingly.
1966		R.E. Alter Carny Kill (1993) 68: I used to work for Madame Esmerelda. She made with the madball.