What kind of substance is marble?

Glass. == I would guess that the first marbles that people played with were round rocks found near streams. Later, they were made out of ceramic materials (see the link). However, the cat's eye and other types are made of glass. There are several hundred different glass designs--with names! The Marble House in Devon, UK is outstanding for its hand blown marbles and extensive collection of marbles available. I was unable to find an online brochure showing pictures, though.