Iris

The Iris ring has a working mechanical shutter mechanism which reveals a hidden stone.  It could optionally hide a laser engraved letter.  The tiny shutter vanes with special gear mechanisms were specifically engineered to fit on a ring and are laser cut from aerospace grade titanium. 

The ring is 9mm wide and the stone is 8mm diameter.  It can be a lab created birthstone like this amethyst or can be a clear CZ.  Just specify in the Comments section.