Like Search Engine Marketing, the phrase "Social Media" is an all inclusive term referring to any membership website. A membership website is one that gives you the ability to join, create a profile, and directly interact with other website members and friends of yours.
Generic examples of Social Media websites are Facebook, Twitter, MySpace, LinkedIn, Classmates, StumbleUpon and YouTube. There are thousands of others.
Some jewelry specific Social Media websites include Polygon and Instore Magazine.
I urge you to get involved with at least one social media website. You should start with the same one most of your customers are using. You should eventually join another after you have learned some online marketing skills specific to social media.
There are very specific methods you should use for advertising on social media networks. Keep in mind that people join social websites to have fun, and they don't want to be sold to. Your personality in any social media network should always remain professional and you should never directly sell anything other than your charm and your smile.