Sandstone. I found an exposed outcropping of bedrock sandstone near my house and here we go. FTT sandstone. There isn't much variety in these, but they can easily be converted to brushes or black and white at low opacity to add a little something extra to your design. (You can click on the images and get the full size ones from Flickr)
I'm the editor-in-chief of Bittbox.com. I'm a designer and developer by day, and a writer and musician when the feeling strikes. I enjoy vintage advertisements and puzzles with an absurd amount of pieces. Follow me on Twitter.