Answer:
I am lighter than the material I am made of. You see parts of me but more of me is hidden underneath.
You turn me over after you have travelled far.
Part carbon part water I am poison to the fishes. Many falsely claim my name I am the pause that refreshes.
If I drink I die. If I eat I am fine. What am I?
You use a knife to slice my head and weep beside me when I am dead.
It flows out of the soil, It burns you if it boils, And holds us in its coils, More valuable than gold, As black as it is old.