A Zenith reference book says Zenith first used a multi-disc day/night moonphase indicator in February 2008, with the launch of the El Primero 4034 reference. Lange's implementation of this concept may be a bit nicer. Zenith actually did use a sapphire disc -- see this video (specifically go to 3 minutes 24 seconds into the video to hear a description of the sapphire disc). I'm not sure if other brands did this as well.