Aventurine didn't know when he fell in love with you. It just happened. Maybe it's because you were the only person who took the time to get to know him, or perhaps it was just that he was desperate for a friend. Who knows.
To him, you were his best friend. But he wasn't sure if you felt the same way about it. You had many friends; he didn't. Which he found unfair. The slander he got on the daily just because of his roots just upset him. He can't control where he's from, and why does it even matter? He's an Avgin. Big deal. The rest of them were already killed off, so he didn't understand why people took time out of their day to just be rude.
You were the first person to step up against it. You were willing to fight someone for him. That was the start of your friendship. And it really just grew from there. He wasn't sure when exactly his romantic feelings developed, but it's been awhile. He wanted to ask you out a long time ago, but his own self loathing got in the way.
He hid his feelings well. He always has been able to. So you probably never would've known what hid behind that confident mask of his.
But when you told him about your crush, you swore you saw something other than cockiness on his face. Of course, it disappeared quickly, but it was there. You didn't dwell on it. Maybe he was just surprised.
Without even knowing, you would only talk about your new crush whenever you were around him. You failed yo realize how it upset him. He didn't even bother keeping his facade on...how did you not even notice?
Today, you were in a cute cafe with him, chatting away about the progress you made with your crush. Of course Aventurine was jealous. He liked you first; he liked you more. He knew you first. He was nicer. But how could anyone want him? He's an Avgin. They're frowned upon. Maybe that's why you didn't like him back.
You just continued talking about this person. It bothered him so much. How could you not see it?