This is a quick and easy tutorial. The hardest part is deciding which plasma render to use and that is a personal choice.

Get the gems brushes here. Unzip the brushes into a new folder on your hard drive. The brush used in this tutorial is Gems_31.gbr. Copy it and paste it into your /.gimp/brushes folder.

1. Open a new image. I started with a 400x400 transparent image and renamed the Background layer, plasma.
   
2. Filters > Render > Clouds > Plasma. Click the New Seed button until you get plasma with colors you like. I increased the turbulence to 1.3.
   
3. Colors > Hue and Saturation. Slide the Lightness slider all the way to the right (100). I increased the Saturation to 59 on this particular Plasma rendering. It is a judgement call though because every plasma is different. Play with the control to come up with what you like.
   
4. Make a new transparent layer and name it gem.
   
5. Set your FG color to 8b8b8b (medium gray).
   
6. Click the Paintbrush tool and find the Gems_31 brush. I resized it to 0.5. Paint over areas in the plasma that you think would make pretty opals.
   
7. After you have your opals in place, use the Fuzzy select tool and select the transparent area around the brushes.
   
   
   
   
8. Select the plasma layer and press the Delete key.
   
9. Merge the gem layer down to the plasma layer.
   
10. *** The brush is a bit rough. Do this to smooth your opals. Right-click the layer and choose Alpha to Selection.
    
11. In the Paths dialog click the Selection to Path button.
    
12. Unselect.
    
13. Click the Path to Selection button.
    
14. Invert the selection.
    
15. Select > Grow > 1 pixel.
    
16. Press the Delete key, unselect, and you are done.

*** Steps 10 through 16 are a bit tedious but Path to Selection is smoother than doing Alpha to Selection.

This is an opal pendant I made using the opal plasma tute by Oregonian.
I made the opal resized it twice to make the trio.
I used Oregonion's silversparkle to make the borders around the opals.
I then made the chain on a transparent layer, filled with the same silversparkle then found a clasp on the internet and filled with silvesparkle also on a separate layer.

Those look so real, Molly. I'd love to have a necklace like that. Thanks for trying my tutorial.