Our 20 pack of steel marbles is designed for use with the popular GraviTrax Marble Run game. Use to replace lost colored marbles or use as extra marbles when expanding your track and racing. Set includes 20 total marbles: 5 green, 5 red, 5 blue and 5 silver. Each marble steel ball is ½” in diameter also making them the perfectly sized option to replace marbles used with many other popular games and marble runs. These steel balls will work on most marble runs and marble towers and with the solid steel construction offer a heavier weight ball than a traditional glass or plastic marble, leading to “faster” runs down the marble tower. The marble size is also great for replacing lost marbles with Gravity Maze, Brio Labyrinth and other wooden marble maze games, Brio Pinball and other pinball ball games and SpaceRail Marble Rollercoaster. Our marbles are engineered for long-lasting play and superior surface quality for optimal performance and lustrous finish.