Gygesringtone2/5
Fully diminished chords, generally only show up in minor keys, but can be borrowed by major keys. They are based on that 7th note of the scale, and are a stack of 3 minor thirds. There's two very cool things about this chord, one is that it's totally symmetrical, you can put any note of the chord on the bottom and still just have a stack of thirds. There's no way of telling (by ear) which note is the root of the chord outside of a larger harmonic context. Also if you stack four minor thirds, you get a set of two tritones.