It's the Amethyst Citadel wallpaper--except in the daytime! I still like the night one better, but it's not bad. The (accidental) openings in the crystal are obvious now; that's how you sail into it, I guess. The citadel's material actually required quite a bit of tweaking to make it look right; it wasn't quite as simple as flipping on the daylight and adding some clouds.

Tools used: Bryce 6