We all assume he’s talking about Hamlet’s heart – that it’s Hamlet he’s referring to here. But it could well be his own heart. After all, Hamlet is dead. The cracking of his heart is in the past, if it cracked. Now – it is Horatio’s turn to mourn, his turn to feel heartbreak. He has threatened to go with Hamlet to death to avoid this pain but has, at Hamlet’s request, elected to remain and draw his breath in pain. Of course he is heartbroken.
But it would be a BIT odd for Horatio to describe his own heart as noble. That’s why we assume it’s Hamlet’s heart he’s talking about – because why would Horatio describe his own heart thusly? But…it’s possible Horatio’s heart has been ennobled by the events of the play. Or his heart, having been given to Hamlet in some way, is now no longer his. It could be Hamlet’s heart in his chest. But of course – it might still be the standard interpretation of this line. That it’s a fancy way to say Hamlet is dead.
But somehow, for me, it might be powerful to have Horatio be describing what’s in his own chest. It becomes a struggle then. It becomes a moment of feeling rather than a report of the facts it is sometimes.