Location for gem farming in Final Fantasy: The 4 Heroes of Light for players wondering where to go to farm gems, from Rubies to Aquamarines. I wanted to include tips for Amethysts and Diamonds in the video too, but it didn't fit. I'll try to cover everything in the description though.
Basically, the places in the video are the ones I use for gem farming in this game. You might find some places that work better for you, which is good - I just want to share the locations that I visit for those who don't know where to look.
What I suggest for gem farming is having 3 Merchants with Finder/Keeper, and either a Hero with Bladeblitz and a good blade equipped or a Wayfarer with Escape (if you don't want to gain levels while farming for gems). Have the Merchants use Keeper on the first turn, Finder on the second, then either finish the enemy party off with Bladeblitz or run away. If your armour is too weak to withstand attacks here easily, you might want to have a White Mage in the party instead of one of the Merchants.
Do not that if you upgrade your equipment properly, you don't have to fear enemy levels increasing together with yours.
Using the Hunting Horn to summon monsters saves time you'd otherwise waste running around, but it's not really necessary, it just saves you time.
This video assumes you've defeated Rolan and have air transportation with the help of your dragon friend. There isn't a strong necessity to gem farm before then, as the equipment in stores keeps getting better and better and it's what you buy and what crowns you pick that determines your success at the early point. After the darkness though, equipment upgrading slowly becomes pretty much compulsory, especially if you overlevel beyond the enemy's starting levels, making the game harder for yourself.
I think Mount Gulug (north from Arbor) is the best place for this. All three enemy parties drop Rubies, and you can get Topaz and Lapis as well. It seems to me that the rate is 50/50 when an enemy has two gems to offer, but I'm not positive on this. Equip Flame Shields to have an easier time here.
Emeralds & Topaz
Quicksand Castle (East from Guera) seems like the best place, as all three enemy parties drop either Emeralds or Topaz. Equip Rock Shields to protect from most attacks and spells here, but what's most important is that you don't get petrified. Have Ribbons on or, if you don't have any, Petrify Capes or whatever they were called.
There's a good spot a little south from Arbor on the overworld map for this. Alraunes offer Sapphires/Aquamarines, but Giant Moth parties drop ONLY Sapphires. Awesome. Protect yourself from sleep and confusion, easy if you have farmed 4 Ribbons for everyone.
Tower to the Skies inside Urbeth is a good place for this. Just avoid the Orc Kings who only seem to give Rubies, unless you're also interested in those.
Any icy area works, really. The one I'm more fond of is on the overworld map outside of Invidia. You might also get Sapphires and Topaz in the process. Ice Shields rock etc.
This one's trickier. You'll always get those from Metal Flans, which can be rarely seen in any area. If you want to get many of these quickly, wait until the bonus dungeons and farm there. Unless you plan to master all the crowns, the ones you get during the story should be enough though. It also works to farm a boss with a Freelancer in your party and 3 Merchants, die (without losing any gems) and repeat. Rolan is perfect for this, as he's only 2 screens away from the adventurer you're sent to upon dying. User wharcraff also suggests to farm Amethysts from Behugemores, found in the Ice Caverns near Invidia in the game's second half.
Second-half of the game bosses drop those, beginning with Rolan. Once again, if you want to master all the jobs, farm him for 99 of these. If you've already beaten Rolan, any of the demon lord's servant bosses also work, as does the Ogre Bear, or even the final boss if you're really desperate. Later floor bonus dungeon bosses also provide Diamonds.
Hope this helps.