Synthetic corundum has been produced since the turn of the century.
During the war it (and beyond) it was used as “jewels” (bearings) in
fine watches and other delicate machinery that required the wear
qualities of corundum.
ou can’t really “add carbon” to it; the reason the tester is testing
diamond is because it’s either a lousy tester or being used
incorrectly. it’s a common occurrence with some brands.