This tutorial is available as a PDF. Click here to view or download it
I am almost hesitant to post this tut because it is so simple, but a search found no marble tuts covering veined marble. So, I will go ahead and post this tut for better or for worse.
This is what we are aiming for in this tut...
1. Begin with a New Image 800x600. If you are into naming your layers, you can call this layer "Base" (or Bob, or Wallace, or whatever you want).
2. Render Clouds using default colors with Scale @ 600 and Rougness @ 0.60. Set the Blend Mode to Difference
Effects > Render >> Clouds
3. Now hit Ctrl+F to duplicate the render clouds effect. You can do this as many times as you wish. To get the effect pictured above I hit Ctrl+F five times. I have found I get best results if repeat the render an odd number of times. You should end up with something like this...
4. Duplicate the layer. Name the duplicate "Overlay".
5. Now change the Levels setting for this new layer. Set the top Input @ 35. Everything else can be left at default.
Adjustments > Levels
6. Set the layer Blend Mode to Overlay.
7. Create a new layer, name it "Background", and move it to the bottom.
8. Choose the default light gray from the color pallette (HEX: 808080) and bucket fill the Background layer.
9. Choose your Base layer and set the Opacity to 50
10. Flatten your image and, voila, marble...
Play around with Curves (or your favorite coloring method) to get various results
Adjustments > Curves or Curves+
I hope this wasn't too simple for a tut. I know it would have helped me out to have one rather than fiddling around until I got the right (or at least satisfactory) result (but, hey, most of the fun is in fiddlin' around).