I think this is a good idea

[Aqualys]

Blue Gemstones are already buyable from NPCs, and the other two aren't. If you could trade 1 for 1, it would kill the point of having NPCs sell just blues, and not red and yellows. It would make it too easy to use and spam those rarer skills that require yellow or red gems (Hocus Pocus, etc).

However, I personally feel that being able to trade 1 red for 1 yellow or vice versa would be acceptable, but only by removing Jade beforehand (or you'd get a yellow gem for two blues by trading two blues for a red, and the red for a yellow).