What's the best Nikon lens for jewelry photography?

Product photography of jewelry is not optically tricky. The kit lens you already have will do nicely. No need to rush out and buy a 50mm. That said, a 50mm prime with a large aperture, say f/1.4 will deliver shallow depth-of-field when set to maximum or near maximum (wide-open).

The real trick will be cutting the glare radiating from polished metal and glittering gemstone. For this, you need “tent lighting”. We cover the jewelry with a white sheet and light through the sheet. The idea is to totally diffuse the light so that it has no visible source (scrambled). This cuts down the glare. Jewelry will look unimpressive with such flat lighting. Now we add a point source we can direct. This point source produces specula highlights that bring life to jewels.

Bottom line – you need to lookup jewelry lighting and practice. As to optics, unless your goal is to make giant prints, the lenses you have will do just fine.