glass marble; marble stopper in a ramune bottle
Specifically the glass marble that seals a bottle of ramune (Japanese lemon soda). Also used for the marble itself as a toy or collectible.
See also: ラムネ
Push the glass marble to open the drinking spout.
When I was a child, I used to collect ramune marbles.
Compound of ラムネ (ramune, from English 'lemonade') and 玉 (ball, marble).