What does an 595 stamp on a piece of jewelry mean?

It almost always means that the piece of jewelry is Gold Plated over a Silver piece of Jewelry

I work in the jewelry business and the above answer is incorrect. 585 is the stamp for a peice that is 14K gold. If it is plated it will have additional letters such as GE for gold electroplate or GF for gold filled. If it is stamped 585 P that stands for plum and has been tested to be 14K solid gold.