Raised in the west of Scotland in Dumbarton, Alexandria-born Diamond graduated from the youth team at Aberdeen in his first full season and made his League debut against Dundee United at Tannadice, coming on as a substitute at half time in 2003–04 season. He made his first start for the club in a Scottish League Cup match v Dumbarton in September 2004. Later in the same season, he scored his first senior goal for the club in a 3–1 win against Kilmarnock at Pittodrie. He won the "Young Player of the Month" award for February.