After you meet Jehk, he tells you that Sage Joch is on the Gold Isle. After you get the key to the Golden Tower from Mara in Southtown, you unlock the Palace of Light.
Explore the Palace
In the first room of the Palace of Light, you encounter two Beast Zombies. There is also a chest here that contains a Spear's Orb. Go north through the door. There is a Dark Ninja here. To the east, there is a Wizard Eye that spits out Weepy Eyes. East of that, there is another Beast Zombie.
When you have gone all the way east, go south through another door. There is another Beast Zombie here. Go up the stairs to the left.
You find yourself in the boss's room. This boss looks a lot like Elinee's Spiky Tiger. The spells that Blue Spike can use are Fireball, Fire Bouquet, Moogle Glare, and Acid Bubbles. Blue Spike has no specific elemental weaknesses, but try Earth Slide, Thunderbolt, Exploder, or Dark Force. Beware: after losing enough HP, Blue Spike will pretend to be defeated by seeming to explode. Don't be fooled: if you can still move, you still need to defeat Blue Spike.
After the battle, you receive a Boomerang's Orb. However, you are not done with this palace yet.
Go up the stairs that appear, and open the chest here to get an Axe's Orb.
Go north through the door, then east. You'll encounter a Dark Ninja and a couple of Beast Zombies. When you reach the east side, go south through the door. There is an Emberman here. It can create copies of itself. In this room, go up the stairs.
You now face another boss: the Gorgon Bull. This one can use Earth Slide, and also has a number of physical attacks. Use Thunderbolt.
After the battle you get a Bow's Orb.
Now you are actually done with the Palace. Go upstairs and talk to Lumina. She will give Primm her magic. Popoi does not receive any spells from Lumina. Go up to the Mana Seed and seal it with the sword. Now you can use the Magic Rope to leave the palace.
Level Up Lumina Magic
Only Primm gets Lumina's magic, and it starts at level 0. You might want to spend some time leveling up Primm's Lumina magic. Remember to do your magic leveling in an area where you are holding your weapons, because you earn more magic experience points in combat areas.
Look for Joch Again
You didn't find Sage Joch like Jehk said you would, so go back to the Lofty Mountains and talk to Jehk at the top of the mountain. He'll say that Joch went to the Moon Palace, which is in the Sea of Wonders in the Kakkara Desert. Fly to the desert and land on the sand around the big dark area in the northwest part of the desert.
Reach the Moon Palace
If you landed in the correct place in Kakkara desert, you will be in a screen with colorful stars and some Mad Mallard enemies. The area around the sand will be dark and have stars in it. Go north. You'll find a ferry piloted by a Robin Foot named Karon. Walk up to Karon and he will take you to the Moon Palace for free.
As a side note, you can cast magic on Karon to kill him. But he magically reappears if you leave and re-enter the area.
When the ferry stops, go north. There are squid-like enemies called Marmablues outside of the palace. Go north into the palace.
Inside the palace, the entrance is marked with little purple flames, and there is nothing else here but darkness and stars. Go slightly north from the entrance so that you can see the topmost purple flames of the entrance, then go west or east. You should reach a Marmablue that is guarding a crystal orb. Have Primm use Lucent Beam on the crystal orb.
The illusion will disappear and you will be in the real palace. Go north. Talk to Luna to receive her magic, then use the sword to seal the Mana Seed. Now you can leave the palace. You can't call Flammie from the palace, so go south from the palace and ride Karon's Ferry again to reach Kakkara Desert.
Once again, you didn't find Sage Joch, so go back to the Lofty Mountains and talk to Jehk again. This time he says that Sage Joch went to Tasnica, which is west of here.