Wait, so do we know here is the missing gem?
I just picked up the one from the snow grave (peridot).
I have 8 gems and no event occurs...
I believe that's aquamarine you're missing on the right. which is from the WDT, which was painful. Sapphire, from the volcanic island (bring an ice dragon heart) topaz (only accessible after marriage, requires an item from behind the gold locked door in the ruins Blue left you to die in at the start, then take it and use it at the Desert Shrine, and diamond or obsidian(?) on the left.