Answer:
It's in your hand though you can not feel it. Only you and time can reveal it.
I move without wings, Between silken string, I leave as you find, My substance behind.
If you have it, you want to share it. If you share it, you don't have it.
This is rectangular, hollow and has a lid, and where you’ll find it you might wonder, it is just six feet under. What is it?
It can pierce the best armor and make swords crumble with a rub, yet for all its power It can't harm a wooden club.
I’m grown from darkness but shine with a pale light. Very round I am and always a lady's delight.