Fimo is a polymer clay manufactured by Eberhard Faber. Due to its high availability, ease of use and relatively low cost compared to properly manufactured body jewelry, Fimo (and other similar clays like Sculpy or Modello), is moderately popular as a jewelry material among highly DIY pierced people. Most often it is used as a cheap way to stretch piercings - one block of clay could likely make enough jewelry to go from 10 gauge to over half-inch piercings.
As with most DIY body modification practices, using Fimo or other polymer clays as body jewelry carries considerable risks. Though the clay does harden after baking, it does not become inert chemically, and may interact detrimentally with the skin around the piercing it is used in. Fimo should never be used to heal fresh piercings, or in healed piercings for long periods of time (if at all). Many adverse effects, or possible effects, have not been adequately documented to guage the safety of Fimo as a jewelry material.