ジェム [gem] in Japanese.
G-Bike

Obtain: login bonus, one-time completion reward for various event objectives
Use: buy extra items from roulette, replenish GS, use Continue on quests, extend inventory
Type: Currency, Rarity: ★
Description: -
Other: otherwise only obtainable by buying for real money
Grandmasters

Obtain: login bonus, one-time completion reward for various quests, event quests
Use: buy gacha draws, extend inventory, buy character remakes and renamings
Type: Currency, Rarity: -
Description: -
Other: otherwise only obtainable by buying for real money
Pictlogica

Obtain: login bonus, one-time completion reward for various quests, Exa Battlia reward
Use: buy for Moogle mining, replenish Stamina, use Continue on fights, extend inventory
Type: Currency, Rarity: -1
Description: -
Other: otherwise only obtainable by buying for real money
Record Keeper
Buy: - (sell: -)
Use: buy rounds of gacha, replenish Stamina, use Continue on fights, extend inventory
Reward: -
Drop: -
Type: Currency, Rarity: -, Stack: ?
Description: -
Other: only obtainable by buying for real money. Everything Gems are used for can also be obtained with Mythril, which are the in-game obtainable version of real-money currency
World Wide Words

Obtain: login bonus, various one-time completion rewards, Coliseum reward
Use: buy gacha draws, replenish Tickets, use Continue on fights, extend inventories, buy various consumables
Type: Currency, Rarity: -
Description: -
Other: otherwise only obtainable by buying for real money
Category: Item