[TwoShedsJackson]

I would imagine that the button either had 55 or a diamond flush draw - if he had the two fives it's one of those hands where you're going to end up losing your stack as he got the one card he needed on the river, which also gave you a near-unbeatable hand.

On the other hand if had the diamond flush draw then your mistake was slowplaying the set, then underbetting the turn. He gets his 4th diamond on the turn and your bet gives him 3.5 to 1, probably close enough in his mind as he figures to get a much larger bet off you on the river.

If he's not the sort of player who even considers pot odds then he has no problem min-raising and hoping for the best. Had you a read on him?

On the end he hits the flush and goes all-in like most players would, your hand is quite well-disguised I'd say. I'd have bet the pot on the flop and turn.