When I decided to go back to school to study geology, I really had to start at the beginning with the upper-division undergraduate courses, since my previous education had been in computer and software engineering. The first class I took was Earth Materials, where I learned to recognize various rock types and incidentally fell in love with petrology. We studied a lot of hand samples, and during finals week I took some photos of my favorites. I really wanted to use them as computer wallpapers, but I hate tiled wallpapers that repeat awkwardly. So I fired up a photo-editing tool called The Gimp, and made smoothly-repeating tiles that wrap both horizontally and vertically.