This could be way off but it looked to me like it might be some kind of manufactured decorative rock for an aquarium.
Starting a google search with something like "aquarium glass rock," I found that there's also something called "slag cullet glass" that is a by-product of some kind of industrial production.
My quick random google search led me to also find that a lot of that was done in the upper peninsula of Michigan in the late 1800's with the glass mixing with rock in nearby lakes and perhaps something of a collector's item when found now.
So there's some more confusion for you. Maybe an idea or two of what to start a google search with though, or not!
That is beyond fascinating. It's clear the both set at the same time, or alternatively the green one was hot and dissolved the other's top layer causing clumps to bubble up. As one layer has bubbled up through the other as it solidified. (the white doesn't look like the kind of material that forms by solidifying liquid though.) What's more the green must have solidified fast as the lumps haven't had the chance to rise to the surface. Probably relatively similar densities also.
Surfaces are unreasonably smooth suggesting man made, and you look to have choncoidal fractures in the green stone suggesting it is glass.