1.6 centimeters!
A glass marble is a small, hard glass ball used as a playing piece in children's games. A standard marble is about 1.6 centimeters (5/8 inch) across. The explorer's picture shows a clear glass marble with a twist of colored glass inside.
Marbles are made in several sizes, so the clearest standard comes from the rules of the game. The National Marbles Tournament, first held in 1922, is played each summer in Wildwood, New Jersey. Its rules call for 13 identical target marbles made of glass, each a standard 5/8 inch across. That is 15.9 millimeters, or about 1.6 centimeters.
Each player also has a shooter, the marble flicked at the targets. A shooter can be glass or stone but not metal. It must be between 1/2 and 3/4 inch (1.3–1.9 centimeters) across, and shooters are checked for size before the tournament begins.
The tournament game is played on a concrete square 12 feet (3.7 meters) on a side, with the circle painted in the middle. The 13 target marbles are set in an X at the center, 3 inches (7.6 centimeters) apart. Players shoot in turn from outside the circle, with at least one knuckle of the shooting hand touching the playing surface.
A player scores one point for each target marble knocked out of the circle. A player who knocks one out, and whose shooter stays inside the circle, shoots again. A game lasts six innings, and a player who knocks out seven or more marbles in the first inning wins at once.
The earliest marbles were made of stone or clay. In the middle of the 19th century, a German glassblower invented special scissors for cutting hot glass, which made glass marbles affordable for the first time. Machines later made them faster and more cheaply still. In the United States, the M.F. Christensen and Son Company received a patent for a glass-marble-making machine in 1905.
By the 1920s, the Akro Agate Company of Clarksburg, West Virginia, was the largest marble maker in the world. Marbles joined the National Toy Hall of Fame in 1998. US federal rules require a choking-hazard warning on marbles made for children aged three and older.