Google have applied for a patent in regards to their augmented reality-based 'Project Glass', the patent in question relates to a finger ring that controls technology such as Project Glass.
The patent is described as a "wearable market for passive interaction", in the form of a ring, artificial fingernails or other wearable items that could interact with a head-mounted display. The technology would reflect infrared light, which isn't visible to the naked eye.
A single camera could determine two-dimensional positioning, backed up by a pair of cameras that would detect three-dimensional positioning. The heads-up display (HUD) would sport menus, and would be controlled by the marker's position or specific gestures. This patent would definitely make sense, especially for things like navigation, and more.