Aluminum memory disks. I think the largest were 48" dia. I first saw them in the late 1960's. The magnetic memory layer was either electroless nickel or cobalt. The gold on top was a protective layer. They tried to use platinum group metals instead of gold but, after sitting around for a period of time, organics in the air tended to polymerize on the surface and formed a gummy layer that looked like rubber cement under the scope. You could move the gunk around with a small probe. A guy that later became my partner was using rhodium plating for these, probably because of its hardness and durability. He supposedly had the largest rhodium plating solution in the world, at that time.
If I remember right, the gold on those disks is not very thick.