When Stone Retrieval Unit enters, create a tapped Powerstone token. (It's an artifact with ": Add . This mana can't be spent to cast a nonartifact spell.")
When Stone Retrieval Unit enters the battlefield, create a tapped Powerstone token. (It's an artifact with ": Add . This mana can't be spent to cast a nonartifact spell.")
Automatons could be rebuilt. Powerstones could not.
