The tooth was a bone given to Richard by his father, taken from the beast he killed while retrieving the Book of Counted Shadows, to prove to the rightful owner that the book was taken to be protected, and not simply stolen. The beast had been placed there by Darken Rahl, who had intended for the beast to guard the book until he could retrieve it.
The tooth was triangular shaped and three fingers wide. Richard had strung a leather thong to it so he could wear it around his neck. The tooth came from a beast from the Underworld as it protected Richard from the beings in the Boundary. As Darken Rahl had used magic to send the beast for the book's protection, he could find Richard by the tooth as it was still linked to Darken Rahl by his magic.