Obsidian Butte at the southern shore of the Salton Sea illuminated by a setting 16% moon (out of frame).
I had hoped to shoot the Milky Way here, but the light pollution was far worse than I had anticipated.
Obsidian is produced when magma cools very suddenly so that there is no time for mineral crystals to form. The resulting rock has a structure similar to glass. In ancient and modern times obsidian has been honed into ultra-sharp arrowheads, knives, and cutting tools.