We've seen him with Diamond Eyes before. (This isn't even all the instances, and only from Season One and the only one's I actually wrote down in Google Docs, add more if you have them!)
Episodes and why he got Diamond Eyes:
Monster Buddies: Steven gets diamond eyes when he's hugging Centi and again when he falls to his knees towards Centi's gem. And again when he says "but if she couldn't do it..."
An Indirect Kiss: He gets stars in his eyes but then reflects diamond eyes in the reflection of Amethyst's gem.
Mirror Gem: Steven wants to relate to Connie so he gets excited when Pearl mentions that the school day had ended. Steven reacts to this by getting diamond eyes.
Space Race: Steven gets diamond eyes when he thinks about how Pearl would get so happy to see space again.
Island Adventure: He gets diamond eyes upon seeing the warp pad again after be stranded on an island for days/weeks(?)
The Test: Gets diamond eyes when he gets the card to roll the dice to escape the eggquake.
