After you have collected all 50 of the Red Jewels in the game, speak to Gem to go to the Jeweler's Mansion.
About the Mansion
You won't get any power-ups of STR, DEF, or HP from killing monsters here, so don't worry too much about skipping monsters if you don't need to fight them. Sometimes you have to kill monsters to proceed, though.
The only tangible reward you get for completing this dungeon is an Herb. You also get to fight a boss that originally appeared in this game's spiritual predecessor, Soul Blazer.
Explore the Mansion
Go west. The leftmost Red Flayzer opens the path. Go west. The Blue Eye Stalker opens the path.
Go north past the many enemies. When you can't go any farther north, go east. From there, go south.
In this area, there are two paths on the bottom left. Take the rightmost of the two left paths. Then go around and east over the stairs. From there, go north to find a chest that contains an Herb.
Go south and go past the stairs. Go west, then use Psycho Slider to slide north under the stairs. Go northwest until you find another set of stairs. Go east across them, then go into the door.
If you get too close to the Solid Arm, it will become completely metallic, preventing you from hurting it, and it will start hitting you with its arm. Don't go past the conveyor belts or you will not be able to damage the Solid Arm.
There are three conveyor belts in the Solid Arm room. The center conveyor belt moves toward Solid Arm, while the other two move away.
If you use the correct pattern, you can avoid damage completely. Walk slightly to the left at the start of the battle so that Solid Arm starts slowly walking instead of spitting fireballs at you. Then walk up the left conveyor belt just enough so you can hit Solid Arm with your flute. Hit Solid Arm until it stops walking and is about to spit fireballs, then let the conveyor belt push you down, then walk to the right so that you're just under the middle conveyor belt, and then face left and hold L or R to block any fireballs. If you are far enough up and are just below the middle conveyor belt, the fireballs shouldn't hit you at all. Then go back to the left conveyor belt and repeat the process until Solid Arm is dead.
Afterward, you will be sent back to the Tower of Babel.