Here is a little trick to create a great "Grunge Marble Brush" in minutes.
Open a new 300 x 300 white document then go to: "Filter - Clouds" and then "Filter - Difference Clouds".
Now go to: "Image - Adjustment - Invert".
Like it? Well, go now on "Filter - Brush Stroke - Crosshatch" and use this settings:
You'll get something like this:
"Image - Adjustments - Color Balance", play with setting 'till you get your favorite colors.
Right, the main part of our work is done.
Now we'll transform our image in a brush.
Get the "Lasso Tool" (
) and draw a selection on the image.
Now go to: "Edit - Define brush", give a name to you brush, and you're done!
Some tips: if you apply many times the difference clouds filter in step 1, you'll get many different effects, and therefore, many different brush, this, also if you play differently with color balance, or with the choice of the final selection.
More Tutorials: 
