Can you identify this green rock??

It's green Obsidian. A big huge chunk of it

Here's another big chunk

Green obsidian is quite rare. Its a good lithic source indicator for trade of source rock for tools and points. In the US theres a very few spots with it.

Im leaning on the specimen shown by the OP to be "foundry glass" from an iron or chrome smelter , fiberglas plant or even a glass foundry where the mix had gotten put aside for colored glass.

obsidian has striations and is translucent in finer shards. This sample looks really opaque.

You're probably right. But how could he test to know for sure?

Id probably do a bunch of easy tests . Like specific gravity, color and crystallography under polarized light microscope.
The opacity on thin slices is a good diagnosis(unless it IS a glass cullet left over from something like stained glass manufacture).
Age dating is easily done by a geology student proficient in polarized light mmicroscopy.
There aresome fancier tests (which usually arent done because of cost). Secondary Ion Mass Spec would identify the metal components (Green obsidian is usually the result of air , not metals. Colored glasses are made of iron or chrome inclusion.
Id but it up and put it in a tumbler to see the layering close up. I would NOT put it in a mineral saw (diamond blade) because Ive had students really screw up an expensive blade by exerting too much force on the "infeed" and then the saw blade starts plucking diamonds and the whole thing starts smoking and the saw blade will be ruined.

Usually the best way is to just keep in mind where he found it and what was the makeup of rocks there. (Was there an obsidian deposit?) or was this just found in a pile of other rocks and clinkers?

Since obsidian degrades into perlite by taking on water, we dont find any obsidian in really ancient rocks,(Usually no older than K)

PS, that is really dumass dress for someone who's stomping around a quarry. Also, please tell that kid to look up whenever hes near a rockwall. That one looks really unsafe for rock hounding. ALso we woulda kept him on the bus (sandals and shorts and no helmet--JEEEZ)

It's just a photo I pulled off the web somewhere. I don't know those people.

oh, never mind

Came with the house I bought as a decorative rock. So no history really other than the original home owner owned a large "sand and gravel" company. Located in Michigan.

its probably a cullet. There are lots of art glass and industrial glass works in Michigan (due to the clean sandstones that dont require lots of decoloring like the Ohio glass sands do)