Your final Gem quest is much like the first three. Once again you will find the next continent east of the previous one, and once again you will find the castle you need (the The Castle of Shamino) on the inner sea, this time on the north shore. The king will tell you to kill a Balron, which can only be found on the Dungeon Levels 9–10. Doing so will earn you the White Gem and a very nice extra prize—the King will allow you to Get nine items from his storerooms. This works similarly to stealing, except you won't be caught and attacked by the guards. Of course, if you are able to find and kill a Balron, you probably don't need much more gear.
This is also the continent where you will find the The Grave of the Lost Soul, and the King in The White Dragon's Castle will send you back to the Lands of Lord British to find the The Tower of Knowledge. You may want to spend some time visiting landmarks and repeating Strength quests to prepare for the final confrontation with Mondain