currency exchange bug
If you’ve logged out and back in a few times since you posted and you still don’t see the gems, I encourage you to contact support (top left, third link from the left — use the “Ask a Question” option).
If you spent the gold and didn’t get credited for the gems, only Support can address it and only if you file a ticket with them. They’ll ask you about the approximate time and how much gold was involved.
Good luck.
John Smith: “you should kill monsters, because killing monsters is awesome.”