Great Start Babett. The key with light is to make it subtle.. are you using PSP or Photoshop?
One suggestion I have is to use layers to integrate the image and lighting effects.. the spot is too obvious

and you could really punch up your drama levels by using the spot only on the background and then applying the light to the Gem on a seperate layer.
Masking is something that takes a little time but once you get the hang of photomanipulation for digital ink and paint you'll be amazed at what you can accomplish.