I have added three more images this time taken by USB microscope - sorry I couldnt work out how to show the magnification factor. I hope these help the identification. They really have me scratching my head.
Also hardness is at least 7 (scratches glass) and density about 2.5 (I thought it was more until I tested it in kitchen).
Found at a building site excavation in Suffolk, UK. Theser rock/mineral fragments are dense, slightly magnetic,shades of green/blue and "greasy" to the touch. They seem to have sheered cleanly and have an almost "glassy" appearence. Some have internal "debris" in the shape of marbles. Hopefully the pictures will allow identification.
I fear that you (and my wife) are right and not the exciting find I had imagined. But what would blast furnace slag be doing in a rural field in Eye, Suffolk? And would glass/ceramic be magnetic??
Yes - glass slag.
Note that glass does not necessarily refer to silicon dioxide glass; various materials in an amorphous state can be called glass. In this case, the green colour could be related to ferrous iron.
Since we are mentioning magnetic glass, here's once instanace - http://www.britannica.com/EBchecked/topic/357184/magnetic-glass
And here is something more esoteric - spin glass.
I just have to copy the text here, even though it is not actually that relevant, just because it is a nice example of jargonistic obfuscation"
"A spin glass is a disordered magnet with frustrated interactions, augmented by stochastic positions of the spins, where conflicting interactions, namely both ferromagnetic and also antiferromagnetic bonds, are randomly distributed with comparable frequency. The term "glass" comes from an analogy between the magnetic disorder in a spin glass and the positional disorder of a conventional, chemical glass, e.g., a window glass."
...from a very technical Wiki entry - http://en.wikipedia.org/wiki/Spin_glass
I live and I learn but also feel a bit disordered if not frustrated. Any idea what might be the agent that would result in the blue rock (see tape measure picture above)? In natural light it is slightly more blue than the picture suggests.Any ideas also re. the "marbles"? . Given the lack of any heavy industry here is there any possibility that is could be created by a lightning strike? Clutching at straws here.....
Darkish greenish-blue - copper compounds.
Pale blue-grey - not sure of actual elemental cause, but it is a common colour in flints and related cryptocrystalline forms of silica.
Marbles: probably globules of molten something-else, remaining distinct from the glass due to higher melting point.
Not related to lightning strike - fulgurites are very different.
Bear in mind that a lot of slag from industrial processes such as smelting is used for roadstone and other constructional purposes. In that way, it can end up a long way from the indstry that created it.
Thanks very much - all useful information. It is near a WW2 airfield so guess the runways would need a fair bit of hardcore/fill etc.