Answer:
I am the fountain from which no one can drink. For many I am considered a necessary link. Like gold to all I am sought for, but my continued death brings wealth for all to want more.
Walk on the living, they don’t even mumble. Walk on the dead, they mutter and grumble
What is long and slim, works in light. Has but one eye, and an awful bite?
What can you catch but not throw?
Hard to catch, easy to hold. Can't be seen, unless it's cold
Black we are and much admired. Many seek us if they are tired. We tire the horse, and comfort man, and turn White when we've fulfilled your plan.