Hmmm. I wrote the "Ha, ha, ha" reply thinking that "Driftmoon" was intentionally designed to have us fretting about the Gem of Ro as we proceed through the game.
Now, I've read your most recent reply, and apparently, there really is a problem.
Could it have been under a different tab?
I checked all of the tabs thoroughly. Gem of Ro isn't there. Looked at items.ini to determine which picture is used for Gem of Ro (gem4.png), then looked at graphics\items to determine the appearance of gem4.png, then checked all tabs again. Gem of Ro isn't there.
Strangely enough, I was able to finish "Driftmoon" with only the first four gems in my inventory.
(A message to the forum with my thoughts about the positive aspects of "Driftmoon" forthcoming in a day or two.)