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.
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.
I have rivers without water. Forests without trees. Mountains without rocks. Towns without houses.
As round as an apple. As deep as a cup. All the king's horses can't pull it up.
Put into a pit, locked beneath a grate, guarded through the night, yet it still goes out.
This food is a tortilla filled with cheese and sometimes chicken before being heated or cooked.