I agree with the others that it is a natural concretion but there is an early archaic item called a shaft wrench that was usually made from bone. It had a hole in it and a wooden shaft was heated and passed through it, by twisting it against the wood pressure could be exerted and as the wood cooled it would remain in that position. Your item could be used as such, just one side of the loop is open. Check the inside of the loop for wear or abrasion marks, these people were very resourceful and not above using something natural to accomplish their goal.
Just a thought.