This article is from the Piercing FAQ, by Anne Greenblatt with numerous contributions by others.
Instead of a bead, a short, straight or curved tube or solid bar is
held between the ends of the ring. The tube or bar is usually the same
gauge as the ring. Because of the tension required to hold the tube,
this design only works well in 14ga and thicker. Captive tube rings
are inappropriate for new or unhealed piercings because of the seams
between the ring ends and the tube.