I had a business that my group decided was a gold mine and refinery, and that we sold the metal to the Imperial Mint and so I got money in my envelope at the start of each event.
The resource is a way of giving every character some income in their envelope when they start the game, and a way of distributing resources to the players to trade with. There are only a limited selection of options so that PD can actually provide all the necessary items, coins, sculpted resin blocks or cards depending on your options.
What you say your business, farm, mine or mana garden looks like is entirely up to you and not important to the game design as such. A precious metal or even precious stone mine would on this system be best represented by a business. Exactly how your metal or stones are converted into cash is below the abstraction layer, and good for a short story over a cup if you want!