There isn't much information about this Nintendo Gameboy ring around the internet. That said, it appears to be made of clay (possibly) and from what we see in the photo found on flickr, it has plenty of detail although it's safe to assume it doesn't actually function as a Gameboy... but you already knew that.
As far as I am aware the Nintendo Gameboy ring is not for sale and was custom built by someone who just loves the old handheld games machine.
Via: Craziest Gadgets and Flickr