mystery "rock"

[Betty Boop]

Any ideas about what this "rock" is? Glass-like and very shiny, pretty lightweight, lots of air bubbles, looks black but upon close examination it's dark green/brown, shows a few layers, found on the Delaware Bay of New Jersey beach yesterday. Thanks!

 

[Indian Steve]

Looks like slag to me.

 

[gtsatohms]

Ditto. Location would be right for slag glass.