A while ago I extended my vintage Constellation collection by acquiring an 18K example with the slimline 712 movement. Although it was designed as a dress watch it is still a full chronometer movement made by Omega, so I figured it had a place in a representative group of Constellations made in the 50s and 60s. The movement serial number places it in 1968, but unlike all my other Constellations, this 712 is fitted with a mineral glass (or sapphire - I can't tell the difference without scratching it) rather than the usual acrylic, which raised the question of when Omega first used mineral or sapphire glass in production. I confess I was a little surprised, as I had assumed that mineral/sapphire glass was not introduced until the 1970s. Does anyone have an answer? A few pics for those who are interested.