Answer:
It's in your hand though you can not feel it. Only you and time can reveal it.
We are all around, yet to us you are half blind. Sunlight makes us invisible, and difficult to find.
Salty water everywhere but not sea in sight.
In marble halls as white as milk, lined with a skin as soft as silk. Within a fountain crystal-clear. A golden apple doth appear. No doors there are to this stronghold, yet thieves break in and steal the gold.
Who is that with a neck and no head, two arms and no hands? What is it?
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.