They are amorphous or micro-/cryptocrystalline; that is evident from the (albeit tiny) conchoidal fractures.
That makes flint or chert a possibility, but the clarity of the clear parts rules them out. Most likely they are glass. But glass can be natural, eg. obsidian, or man-made. I don't think it is obsidian, but it would be useful to know where you collected the pieces, as an aid to ruling obsidian in or out.
While I await your clarification on that, I will suggest they are pieces of glass waste.
The clearer parts would have been relatively internal to larger pieces your bits broke off from, and being internal has prevented those parts from devitrifying. Exposure to the atmosphere and/or soil has allowed outer areas to change composition (structurally if not chemically), losing its glassiness - ie. devitrification.
Any shapes you see are incidental, not fossil.
Maybe there were glassworks in the area?
However, that does not necessarily follow, as waste from glassworks and other industries can be transported far afield and used in road building and other ways.
found near a coastal quarry in northumberland amongst a serious amount of flint and a few pieces of what looks like obsidian.
i have checked the local history for glass works and found nothing.
Useful further info, thanks; nothing we can really hinge our ID on, though.
Northumberland certainly has volcanics, which may include obsidian (though the age of some of those rocks is too great to expect obsidian as such; it would have devitrified by now).
And the lack of awareness of a glass industry does not prove there was none.
Sometimes, we are just short of information.
We are not the first ones to have trouble making a firm ID of this sort of thing, mind...
Back in the 19th Century I note this paper:
WOOD, W. H.
'On a Chemical method of distinguishing Black Obsidian from Black Blast Furnace Slag',
Proceedings of the Yorkshire Geological Society, Vol.VII, p.274
While we are mentioning obsidian, this is an interesting page
And this is a useful account of rock types in Northumberland, from a building stone perspective
It does not mention obsidian, but it does mention trachyte, which can occur as glassy forms.
On balance, I still think it is glass slag.