I'm new to this but I had a close friend design and print the Ring of Gyges (the invisibility ring from Plato's Republic); the ring also featured in my story "Ripper's Ring" which will launch at http://www.gypsyshadow.com/StevenSouthard.html#top on May 1, 2015. The triangular 'stone' rotates. When the stone's pointed toward the base of the hand, you disappear, and you reappear when you rotate the stone toward your fingertips. Haven't been able to make myself actually vanish yet, but I'll keep trying.
One file makes the base of the ring, and the other makes the stone. My friend glued a machine screw and bolt inside to enable the rotation. You will likely have to scale the design so the ring fits your finger.