A direct quote from the linked article
Dungeon Keeper's in-game currency cropped up in the response. The game uses two grind currencies: stone and gold, which you get by playing the game. There's also a premium currency called gems, which most free-to-play mobile games use. You can buy these with real money or earn them in the game.
EA said that because players can earn gems by digging out dungeon areas, winning achievements and timed event raids, it was possible for a player to accrue all three currencies for free over time just through normal gameplay.
Of course, if you want to speed up the process you can buy gems, but EA said because gems were regularly awarded through interaction with the game it was possible to unlock the same content through gameplay.
Take a look at the bolded part, sound familiar to any recent Bioware Statement about DAMP's in game currency recently? fun that a studio that is owned by EA is talking exactly like EA was when it tried to defend its micro transactions in Dungeon Keeper