First I would grab a few nice source images like the ones below:
(these are from cgtextures)
After selecting my source images I would then cut bits and pieces from them and arrange it all into my texture. Burning and Dodging was done do get things looking the way I wanted. I also used the sponge tool to pull some of the color out for the next step.
The final step would be to blend this nice marble image over the top of the texture to give it color. I did this to every texture to keep things consistent. The blending mode used was linear burn at around 30-50% opacity depending on the image I was blending it into.
(this one is also from cgtextures)
The end result:
