Maybe approach it like a mechanic does when trouble-shooting--try to figure out what it's not.
So, if it's light for its size--it's not platinum. It's also not gold, due to the weight and the color.
Since it's gray, but not heavy, it's not lead.
Next, you may want to read up on how to do a streak test (for minerals) on the Internet--that may help eliminate other things.
What are the nuggets' electronic signatures (detector response) like (highly conductive, etc)?
If you rule out the heavy materials, that leaves the lighter metals (tin, aluminum, pot metal, etc.).
I've found all kinds of things while dredging: bits of aluminum, pot metal, brass, copper, natural and refined lead--almost everything imaginable got into the streams during the gold rushes. If the river is subject to wide swings in flow fluctuations, the stream materials could take almost any malleable metal and hammer it into a nugget.
Good luck, and I hope you find out what it is, and if you do be sure to post your findings.
All the best,
Lanny