Definitely snag a macro lens. If you can swing the $$$, the Sigma 70mm f/2.8 works excellent. I shoot Jewelry & Gemstones all the time, it is my Wifes other business, she sells retail as well as wholesale and sets the stones as well.
You will quickly find out that this is one of the most difficult forms of photography, shooting gemstones & jewelry.
To be honest, I have found that a nice P&S camera tens to work a bit better for this type of work due to the better DOF.
The most important thing is lighting though.
I actually wrote up a small tutorial. goto
Gems By Jennifer About Me Page and scroll to the bottom of the page, there is a link to the tutorial.
Here is some of the shots I have done, some are with P&S and some with a dSLR.
Jewelry & Gems Photo Gallery by JameyS at pbase.com