Answer:
What is weightless but it needs two person to hold it?
Used on a diamond and left on a board. It's great for a gymnast his grip to restore. What is it?
What does everyone know how to open but not how to close?
What is often returned, but never borrowed/
It's usually found dried, but often kept slicker. Above another need, it's said to be thicker. What is it?
I am your mother's brother's only brother in law. Who am I?