Qualifications
The first—and only—certification you'll need is the one from the Gemological Institute of America (GIA). Without it, no reputable jeweler will hire you—and considering how very ugly the world of gemology can get, it's a good idea to work for a reputable jeweler.
The GIA offers classes in New York (as well as more exotic locales, like London and China), as well as online courses. Once you've got that baby hanging up on your wall, you're all set to start sending out those résumé .
You'll need to have good eyesight to be a good gemologist, since your job will be all about looking at stuff. If you can't tell the difference between a rock and a gemstone, or see that small inclusion that changes the stone from a VVS1 to an SI1, you'll be in big trouble.
You'll also be in big trouble if you struggle with geometry, since good gemologists not only know their rocks, they know their angles. Math really comes in handy when you're checking out angles and figuring out carat weights, percentages, and pricing.
Some people skills will come in handy, too, since the point of creating beautiful jewelry is to sell it. Being able to chat with customers and explain technical things in an accessible way will make your job a million times easier. It'll also help to take a cue from DeBeers Diamonds—the ones with the "A Diamond is Forever" slogan—and use a little psychology to close a sale.
The DeBeers ad campaign cut to the heart of what's at the basis of any good relationship—insecurity, and the desire to prove your devotion to your soon-to-be fiancé by putting a price tag on it.