Refers to the small glass ball used in children's games. The word is derived from ビードロ (vidro, 'glass') + 玉 (ball).
See also: ビードロ · ガラス玉 (ガラスだま)
The children were playing marbles in the schoolyard.
このビー玉はきれいな青色をしている。
This marble has a beautiful blue color.
When I was a child, I used to play marbles in the park.
You flick the marble with your finger to hit your opponent's marble.
He likes collecting colorful marbles.