10. Duplicate this channel and go to Image > Adjust > Levels again. (Shortcut: Command-L). This time, darken the image by moving the Gamma slider--the middle triangle under the histogram--to the right.
11. Continue duplicating and darkening each successive channel until you have isolated the core of the veins. (This core will be the darkest part of your marble.) As you darken each channel, you may occasionally find it helpful to move the Black Point slider as well.
When you are done you should have several different channel masks. You will now use these masks like friskets to apply successively darker colors to your marble.