The oldest marbles were handmade by rolling clay, carving ivory or grinding stone. Glass marbles can also be made by hand in a process that resembles making seaside rock. Molten coloured glass is rolled into rods and stacked together to form a pattern.
The glass is then cut with special scissors, while it is still soft like toffee and the sections are rolled into balls. Mass-produced marbles use multiple nozzles to combine streams of liquid glass that are cut into even-sized lumps and passed between two parallel, rotating screw threads.
As the screws rotate, the marbles are moved along the production line, constantly rolling to form into perfect spheres as they cool. Fancier marbles are sometimes coated with powdered glass of other colours as they roll.
Answered by Luis Villazon