Glass gems and little fish...

[Phether]

If your betta can see its own reflection in the marbles, it might have been trying to fight the marbles and got itself stuck there. If thats the case then it may be a good idea to try and cover it up so your betta can't see itself.

[ElmoFish]

I've mixed some gravel in with her glass gems, and so far so good, she hasn't tried to squeeze in there anymore. She's really curious, so she's always trying to squeeze into any gap she sees, and she's so tiny that she actually fits.

[veganchick]

some bettas loooooooove to dig, and If you have one of those, than I would reccomend using sand.