Powerstones are tied to an Era (Eras last 56 days, or 4 Seasons). If you take a look at any Powerstone in your inventory, you will notice it tells you when it will expire.
At the end of the Era, all obtained Powerstones will be converted to Gold, regardless if they are attached to units or not. Commons grant 10 Gold, Rares 25 Gold, and Mythics 100 Gold per Powerstone. Maximum total reward from Powerstone Reimbursement is 25000 Gold.
With every Era, we bring in a new mix of Powerstones collect that give players access to different types of enhancements and shake up the metagame and strategies used by players. This is a way for us to bring some excitement and new ways to play the game every Era.