A steelie is a type of marble made of steel or a similar shiny metal.
Steelies are often used in games and can be very shiny and cool.
Examples:
I found a shiny steelie in the playground.
Let's play a game with steelies.
The steelie rolled across the floor.
I wish I had a steelie as shiny as that one!
The steelie was so shiny, it looked like a little mirror.
I won the game with my lucky steelie.