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.
I am seen in the water. If seen in the sky, I am in the rainbow, a jay's feather, and lapis lazuli.
Come up and let us go. Go down and here we stay
The higher I climb, the hotter I engage. I cannot escape from my crystal cage.
Crooked as a rainbow, and slick as a plate, Ten thousand horses can't pull it straight.
What is always coming but never arrives?