This article is from the Piercing FAQ, by Anne Greenblatt with numerous contributions by others.
The metals used for body jewelry are chosen for their
bio-compatibility, or "body friendly" quality. However, some metals
are more bio-compatible than others due to their specific
compositions, or alloys. Please refer to Part 6, section 6.5, for more
information about metal sensitivities.
Piercing jewelry manufacturing is unregulated in the United States and
largely unregulated in the rest of the world. Reputable manufacturers
will disclose material specification certificates for the metals they
use. Please see section 2A.1a for an article about material
specification and ASTM and ISO standards.