Answer:
I am seen in the water. If seen in the sky I am in the rainbow a jay's feather and lapis lazuli.
I may seem real but it always turns out I was never there in the first place... you only see me during a certain resting stage.
While you were folding a letter you got me. Usually you will do whatever it takes to avoid me but now you can't help but find me.
I am the home for feathery animals.
I am one small little piece of paper yet sometimes hold lots of value. I am all you need to get in to big events but will cost you. I am an important part of travel. And if lost you're not coming.
The more that there is of this, the less you see. What is it?