High Quality Content by WIKIPEDIA articles! A marble is a small spherical toy usually made from glass, clay, or agate. These balls vary in size. Most commonly, they are about inch (1.25 cm) in diameter, but they may range from less than inch (0.635 cm) to over 3 inches (7.75 cm), while some art glass marbles for display purposes are over 12 inches (30 cm) wide. Marbles can be used for a variety of children's games, and are often collected, both for nostalgia and for their aesthetic colors.