Diamonds are, of course, subject to some chemical and physical reactions: those few electronegative ions that can break the tight aliphatic carbon-carbon bond in a diamond will of course react with, damage and ultimately destroy a diamond. Heat a diamond in air or oxygen past its flash point for that gas, and it will combust. Heat a diamond in vacuum or an inert atmosphere past the melting point, and it will melt (IIRC it sublimes but that’s immaterial).
Probably a fresh-formed diamond has a trace amount of carbon-14 in it; as that breaks down, there will be a microscopic flaw at that point; if interior, trapping a nitrogen atom. The percentage of carbon-14 compared to carbon-12 and -13 is so small, however, that this would not significantly damage the gross macroscopic crystal. I’m not aware of any natural diamond-forming processes that have occurred recently enough to leave any diamonds with measurable amounts of C-14 left in them.
Finally, the diamond structure is metastable; over geologic time, it turns to graphite. But this is not a significant consideration unless you have a few billion years to spare to observe the process.