Diamond is expected to finalise a three-year contract after undergoing a medical at around 8pm on Tuesday evening.
The reason for the delay in Diamond signing is unknown, but it is thought that the triple capture of John Sutton, Jamie Hamill and Danny Grainger made concluding his deal difficult yesterday
The signing could be announced today barring any unexpected late hitches, where he may be unveiled along with Kilmarnock's Mehdi Taouil as Hearts assert their dominance as the SPL's third financial force.
26-year-old Diamond played over 200 times for Aberdeen and has been the vice-captain of the club for four seasons.