The cloudy surface?
Some makers frost their marbles with acid on purpose.
For vintage marbles, it usually means they spent time in water, e.g., in an aquarium.
If you have a worn group, that's where I'd put 'em. Yes, they're commons, but I'm not sure how you would describe the condition.
#4, 5, 6 are Vitro, by the way. At least I think #4 is.
#1 is Akro I think. Maybe #2 also Akro. I figure Pelt for #3 (with tiny niggling thought about Vitro.)