I agree nice eye Fire--What is that crud?
A little Laquer thiner will clean it up with out damage to the glass.
It could be some some of that stickey stuff used to hold an object down during an earthquake.
I have found a few "Filled" marbles in my lifetime.
There is a liquid substance that fills glass and makes it look hole--I think that yours is a harmless hold down substance--
Very nice "Brick!! along with the rest of them--the first one puzzles me a bit.
This one along with the rest are hand made imho.
@Chad G. may have some insight here.
Marble--On!!