Answer:
Put into a pit, locked beneath a grate, guarded through the night, yet it still goes out.
Bright as diamonds, Loud as thunder, Never still, A thing of wonder.
You throw away the outside, eat the inside, then throw away the inside. What is it?
Used on a diamond and left on a board. It's great for a gymnast his grip to restore. What is it?
Often held but never touched. Always wet but never rusts. Often bits but seldom bit. To use it well you must have wit.
What is yours but only used by others?