It takes a long time to anneal glass, and the thicker the section the longer it takes-- until finally it becomes prohibitive. It's hard to see how something this big would have been a production item (as a pedestal, seat, etc). As an example, "A 25 inch blank, 2 inches thick, is annealed for 9 days."
If it was just leftover glass (as from the bottom of a crucible), dumped at the end of a season (or when the crucible was no longer usable) I'd expect it to be in sad shape, damaged, with inclusions, etc. But it looks like a nice solid hunk of glass with no defects (aside from the funky surface)...
...though the surface has a sort of tree-bark look to it, which looks like it may be intentional, as does the cylindrical shape.
Too fragile for balast. Rocks are just as heavy and much tougher.
I'll see if I can find out more.
And yes, this would look great in my yard!
Found a story: Largest Piece of Beach Glass Found in Lake Erie to Be on Display at 2019 Great Lakes Beach Glass Festival
https://www.relishinc.com/blog/largest-piece-of-beach-glass-found-in-lake-erie-to-be-on-display-at-2019-great-lakes-beach-glass-festival/Ashtabula, OH is on the shore of Lake Erie.