If you are a Geologist or more specifically a Petrologist or simply love to just collect rocks, this theme is for you! The Macro World of Rocks theme from Microsoft is a 9MB themepack which has some simply wonderful close and inside shots of rocks.
The colors and patterns of photographer David S. Sommers’ close-ups of polished agate, jasper, and other geological specimens suggest alien planets and fantastic abstract paintings. A free theme for Windows 7.
Head over to the Personalization Gallery to download it.
These themepacks can be used only on Windows 7 Ultimate, Home Premium, Professional, Enterprise editions, but you could extract the wallpapers if you like them and use them on your desktop.
Go here if you want to learn how to preview wallpapers, before downloading Windows 7 theme packs from Microsoft.