There are 4 common currencies & resources within Puzzle Quest 3;
Gold, Gems, Food & Ore
They are usually all awarded from opening Chests, completing Battles & Chapters and produced by certain Followers in the Tavern. While all are awarded in similar ways, they each have their own purpose.
Almost every action in the game, requires Gold to complete. From upgrading Followers to upgrading Spells & Armor.
Gems are used to make purchases within the store, unlock items or speed up the progress of a task. Such as sending a Minion off on a Mission to open a Chest, Crafting an Item with a Follower or skip to the next level of the Battle Pass
Along with Gold, Food is used to upgrade your Followers in the Tavern. Giving them increased bonuses and the ability to craft more resources.
Again, along with Gold, Ore is used with Followers to Craft items. Such as Weapons, Accessories, Shards and so on.
Note: Your Followers can be upgraded to carry more Gold, Ore and Food.