dick jewelry
A girl. The expression comes from the fact that many girls are merely symbols of achievement for males. A ring is a symbol of wealth or achievement worn on the finger; similarly a girl is a symbol of achievement worn on the dick. In simpler terms: a girl is nothing more than something hopefully pretty that you wear on your dick, like the ring from above.
John: Man, Robs chick is hot!!
Phil: Yeah, shes one fine piece of dick jewelry
Phil: Yeah, shes one fine piece of dick jewelry