Have you tested the silvery stuff with a magnet? If strongly magnetic - magnetite.
Does your detector, if you have one, react to it?
If no on both tests, it is likely some kind of sulphide.
Do the following tests:
Can a knife scratch it?
What colour do you get if the metallic looking pieces are rubbed against an unglazed piece of porcelain?
It can help to limit the possibilities.
