For barbells, in 80% of the cases, both of the balls/beads will be threaded.

To open your jewel unthread one of the balls anti-clockwise with care.

If it is too tight, do not try to force it. Try using rubber gloves.

• If this doesn't solve it either, leave your jewel in the freezer for about 20 minutes and, right after taking it out, try unthreading it again with the help of rubber gloves.  The cold temperature will shock the metal for a fraction of a second, making it easier to unscrew afterward.
  
  
• Some kinds of piercing jewelry (such as nipple, or industrial bars) may come with a fixed ball. We do this is to make sure that both of the beads look in the same direction. In this case, do not try to force the fixed bead, and always open/close your jewelry from the threaded ball.
  
  
• For ear barbells they mostly come with only one threaded ball. The other end will be glued on. So always, only, unscrew the ball. Do not grab the glued front while unscrewing. Only grab the actual bar and ball to unscrew.