This article is from the Piercing FAQ, by Anne Greenblatt with numerous contributions by others.
2A.1 Stainless Steel
Of the many stainless steels available, only 316L and 316LVM are
appropriate for use as body jewelry. 316L is a low-carbon variety of
316. 316LVM is 316L that has been vacuum melted; the vacuum prevents any air or airborne contaminants to attach to the molecules in the metal, resulting in a more consistent steel. For most people, however, that the steel be 316LVM is not a necessity to heal and maintain a healthy piercing.
316L / 316LVM stainless steel is comprised of several metals including
nickel, to which some people are sensitive. However, the specific
composition of 316L / 316LVM allows for very little exposure to the
nickel molecules, thus reducing the risk of sensitivity.
The jewelry should be polished to a reflective shine (mirror finish),
free from rough edges, tool marks, and wire-drawing lines and pitting
which are present in the surface of the steel when it arrives from the
mill. When polished, true 316L is a white, not gray,
metal. Under-polishing will be most evident along the inside of the
ring where polishing is most difficult.
316L arrives from the steel manufacturer at specified degrees of
hardness. Most jewelry manufacturers use the least-hard (1/4 hard)
steel available for rings. Working the steel and forming the steel
into rings hardens it to some degree. Annealing, a controlled heating
and cooling process performed in a vacuum, yields a more flexible
steel, resistant to metal fatigue. Most piercers and jewelry
manufacturers agree that steel rings should be annealed to some degree
or made from annealed wire because an annealed ring is easier to
safely and quickly manipulate for insertion; pliers are often
unnecessary for manipulating thinner gauge rings. However, the softer
(more annealed) the ring, the easier it is scratched by tools or
simply by daily wear. The trade-off of using non-annealed rings is
that harder rings require tools to manipulate, increasing the chance
of scratching the jewelry during insertion. Very hard rings cannot be
torqued open at all; bending can cause the ring to break or create
fractures invisible to the unaided eye which can trap bacteria.
316L is classified as an austenitic steel. Austenitic steels are
generally non magnetic when annealed, although some may become
slightly magnetic by cold working (bending and shaping).