Answer:
I am the yellow hem of the sea's blue skirt.
White and thin, red within, with a nail at the end. What is it?
What goes trough your head but you forget it like 3 days after?
Sometimes it's silver but also gold. Printed on paper it's a treasure to hold. What is it?
Longer than a decade and shorter than a milennium
Though not a plant, has leaves. Though not a beast, has spine. Though many wouldn't need this thing, It's more valuable than wine.