If it were created before WWII, it would almost certainly be
synthetic sapphire; other diamond simulants like YAG or GGG, were
invented for the laser optics industry in the 1960s.
You would polish sapphire with diamond grit. In fact you can polish
anything other than quartz with diamond grit, including diamond. I
recommend 50,000 grit for final polish, but you will need coarser
grit to remove scratches. The choice of prepolish grit would depend
on what prepolish lap you are using, as some laps embed the diamond
a little deeper than others.