That is the Pentagram of Solomon. It protects the wearer from malicious spirits.
According to the Lemegeton - Goetia
QUOTE
This talisman is to be made in Gold or Silver and worn upon the breast with the seal of the spirit on the other side of it. It is to preserve the Exorcist from danger, and also to command by it
EDIT
Taking a second look at the photo, Im not so sure that it is the same pentagram as that described in the Lemegeton, nonetheless you can be fairly certain that it is a protective talisman.
This post has been edited by Faustopheles: Jan 3 2007, 09:04 PM