Part of the story and unmissable.
After defeating the Lich, you are required to go to Crescent Lake town on the far East side of the map. Here, you'll get the canoe that allows you to travel on rivers.
You can now go to Gurgu Volcano to take care of the Fire Gem, but you can also go directly to the Ice Cave from here (you'll have to sail a bit from Crescent Lake and then go west in the rivers).
Completing the Ice Cave gives you the required item to get your Airship ready. Once done, head south to the small desert area on that continent and watch the little cutscene.