Answer:
A precious stone, as clear as diamond. Seek it out while the sun's near the horizon. Though you can walk on water with its power, try to keep it, and it'll vanish in an hour.
My sides are firmly laced about, Yet nothing is withinΝΎ You'll think my head is strange indeed, Being nothing else but skin.
When I get closer my tail grows longer, but when I go away my tail leads the way.
Three little letters, a paradox to some. The worse that it is, the better it becomes.
What measures out time until in time all is smashed to it?
What is always coming but never arrives?