It's important to have replacement balls for your piercing jewelry, just in case you lose a ball so you don't have to worry too much. It's also an excellent idea to replace the ball and change the look of your jewelry sometimes. 

This ball is made of surgical steel, which has undergone an ionization process to get these beautiful colors. They're available in different sizes and gauges, so you can find the right one for your piercing.

Opposite the thread, there's a beautiful bezel-set gem. If you want to use it as the bottom ball of your belly button ring, keep in mind that this will face down so it won't be visible. However, it will look great as a top ball in other barbells.

In any case, it's always a good idea to keep replacement balls, and if you're unsure about what color you like the most, you can buy more than one ;)