316L Surgical Stainless Steel Threaded Ball, designed to thread onto any 2mm stem (such as a barbell, circular barbell, bananabell, labret stud.. etc).
Threaded balls are used in almost every item of body piercing jewellery, and usually unscrew to allow the body jewellery to be inserted into the piercing. It's very easy to lose a threaded ball, so why not stock up on a few spares?