Both shots I just have an AB800 firing into a softbox, the softbox basically right in front of the piece.

On the first shot the piece is held up with a little piece of clay, then I merged two shots in Photoshop. They are just sitting on a curved piece of thin, white cardboard. No bouncing or anything.

Second shot, it's hanging by a pencil against my bedroom wall. I made just the piece a layer, then put a gradient on the background.

I'm using a Sigma 50mm f1.4 on a Canon 5D.

Any advice on improving? Equipment or techniques? Is the Canon 90mm tilt/shift an excellent choice for jewelry shots?

For jewelry, you'd best be served by learning how to properly light and retouch your shots. No lens will overcome that hurdle.

www.imagingprep.com is the place to do it.

Buy "Light -- Science & Magic" to learn about lighting objects, especially shiny ones. This is a nice start but a couple of things reach out and grab me:
The shiny bit at the top of the first photo is black in one view and white in the other. This is because that's what was reflected. You must control these reflections to make sure the thing looks silver in both views.
Neither shot make the piece look like it has any depth. The white of the piece blends into the white background.
On the second shot, the highlights are blown out on the metallic shapes. These reflections must be controlled so you can maintain saturation of the colors without blowing out the specular highlights.

<Chas>

I think there is something else missing from these shots: they could be taken for something else other than jewelry. They could be mistaken for larger art or even fancy lighting: the chain does look a little like electrical conduit.

There is real detail here and they are certainly interesting pieces...