Answer:
To unravel me you need a simple key, no key that was made by locksmith's hand. But a key that only I will understand.
Used on a diamond and left on a board. It's great for a gymnast his grip to restore. What is it?
What surrounds the world, yet dwells within a thimble?
I am what remains when nothing is left.What am I?
A dragons tooth in a mortals hand, I kill, I maim, I divide the land.
My days are numbered. What am I?