Dana Hunter made some nice pictures of stones-if I might be so ignorant in this case and in this case only-here. I took one of these pictures (the corner of a sandstone pillar) and tried The Gimp for a bit of enhancement to the visibility of the stone’s structure. Could not call it a complete failure, but could be better.
The original picture, reduced in size by 50%
Color Correction by setting the whitepoint
The automatic did not work here, so I had to set the whitepoint manually, somewhere at a very bright spot around the fingernails.
Slight color enhancement
Enhanced the color with the Ultra-Vivid tool from the FX-plugin with 10% opacity. I find that much simpler than fiddling with the settings of the plugin but YMMV, of course.
Layer with Mids darkened and added by value
Contrast enhanced
I tuned the contrast a bit with the contrast/brightness changer of the FX-plugin which uses matrices to do that.
De Amentiae Mundi 
Pingback: Meanwhile, Back at the OSU Geotour… | En Tequila Es Verdad