Metal jewelry is one thing and metal jewelry with stones is more difficult.  What happens is light diffused and controlled effectively for acceptable reflection in a photo will leave most gemstones dead cold.

For this, a pinpoint light might be tried on a stone..or do as advertiser photogs do...take two shots and layer the sparkly stone on the "diffused light" image.  I know, we would like to do it in one shot!

You might try experimenting with things like the black coils of paper in the diffused area and with different diffusing methods, such as even adding thin tissue(like the white kind things are packed in) here and there to see effects on light on different areas. 

Unless you have a "theme" set of some sort, to get the item only a homemade infinity curve helps in the final look.  I like white or off white for this.

Stray light from outside the "tent" or diffused area is always asking for problems. 

Best wishes!             Tom.