Crystal Desert (ガラスの砂漠, Garasu no Sabaku?), also called Desert of Crystal, is an area on the overworld in Final Fantasy Adventure.
After Sumo has been blasted way from Julius' magic, he finds himself left for dead in the middle of the desert. He's rescued by Chocobo who carries him to Ish.
Much later, Dr. Bowow mentions to Sumo that there's a crystal that can be blown up by Nuke in the desert, by doing so will open the entrance into the Cave of Ruins.
Spoilers end here.