Answer:
I have a little pool with two layers of wall around it. One white and soft and the other dark and hard. Amidst a light brown grassy lawn with an outline of a green grass.
I come without being fetched at night hides away as soon as daylight strikes. Although I may look small I am much mightier than what you can imagine.
I am lighter than air but a hundred people cannot lift me. Careful! I am fragile!
I am in the past, never in the future. I don't exist, but have existed. I saw what you saw, and this is what I will ever see. What am I?
They took me from my mother's side where I was bravely bred and when to age I did become they did cut off my head. They gave to me some diet drink that often made me mad but it made peace between two kings and made two lovers glad.
It has no top or bottom, but it can hold flesh, bones, and blood all at the same time.