I mean, they're chemically stable and won't be eroded under any normal conditions. Realistically they'll outlive the world, however many billions of years that is.
Diamonds are nothing special. They're only seen that way, and people pay top dollar for them. Look into the diamond industry and see how that works.
There doesn't seem to be an official Diamond currency unlike Gold/silver/copper/platinum/aluminium/palladium. But is this your answer to the question?
Hi, they are more or less pure carbon. You can make a nice camp fire of them. So no, they don't last forever, neither their monetary value nor their non-material value lasts forever. Regards Gyro
If you're asking about the permanence of the deaths of all those african children who died so that you could have pretty rocks, yes, it's forever :2thumbsup: