Several things can cause a precipitate in coolant. Material from the head gasket usually causes a grey thick material that settles to the bottom. The coolant can pickup material left over from the casting process, which often includes rust, and that can precipitate out. Silicates can also precipitate out, but that would normally be a small amount. Is it possible you ran with too high a concentration of antifreeze? That can also cause precipitates to form.
There are labs you can send this stuff to and have it analyzed, here's one that does oil, not sure if they analyze coolant.
Bummer man, you have my condolences.
My understanding is Blackstone Labs in Fort Wayne is one of the very best in the nation