LB FFB
Red Socket: Runed Cardinal Ruby
Yellow Socket: Potent Ametrine
Blue Socket (Greater than 5 SP): Purified Dreadstone
Blue Socket (5SP or worse): Runed Cardinal Ruby
CS FFB
Red Socket: Potent Ametrine
Yellow Socket: Smooth KingÔÇÖs Amber
Blue Socket (Greater than 5 SP): Misty Eye of Zul
Blue Socket (5SP or worse): Smooth KingÔÇÖs Amber
LB FFB (other)
See CS FFB above.
OR, If you feel like ignoring socket bonuses and only going for Meta Gem Bonus, after obtaining gems for Meta Gem activation, just use Smooth King's Ambers in all sockets. (generally a weak option and used only at lower gear levels)
If you have JC. Replace 3 would be Red/Yellow gems respectively for the DragonÔÇÖs Eye version. This means that if you are SP gemming you use Runed DragonÔÇÖs Eye x3 while Crit gemming you use Smooth DragonÔÇÖs Eye x3.
To explain the difference of SP gemming and Crit gemming you should first think about the fire specs itself. The most important difference in the 2 gemming systems is RNG. By gemming crit you reduce RNG by having more crits. This means you have more consistent DPS than a SP stacker. When compared to SP stackers, The DPS can be lower or higher. This depends on the SP stackers RNG. If a SP stacker gets good RNG, they will very easily outdps a crit stacker. However bad or mediocre RNG will put them even or below crit stackers.
Another key point about how the DPS differs is with movement. Generally when high movement is involved a SP stacker will out dps a Crit stacker, while on stationary fights it comes down to RNG.
So in general: SP stackers can pull more DPS but are more RNG based than crit stackers who are more consistent.