These bags only hold ingredients for Jewelcrafting, but are larger than the average general bag. Only 1 gem bag may fill a slot in a bank at any time.
“Gem bags are special bags that can store minerals and gems. The [Gem Pouch] is a 20-slot gem bag...Gem bag
“Gem PouchItem Level 60Disenchants into: Not disenchantableBinds when equipped20 Slot Gem BagSell...Gem Pouch
“Bag of JewelsItem Level 65Disenchants into: Not disenchantableBinds when equipped24 Slot Gem...Bag of Jewels