No, they are not. They are the product of confirmation bias, magical thinking and often deliberate fabrication (as pointed out in the comments, there can of course be psychological and psychosomatic effects).
The Wikipedia page about the Hope Diamond details many examples of deaths incorrectly linked to it, individuals who embellished the story of the curse for personal gain, and most tellingly finishes:
Since the Smithsonian acquired the gemstone, the "curse appears to have gone dormant." Owning the diamond has brought "nothing but good luck" for the nonprofit national museum, according to a Smithsonian curator, and has helped it build a "world-class gem collection" with rising attendance levels.
People who believe in curses would no doubt come up with an explanation (probably focusing on the "nonprofit" angle) rather than admit that there simply is no curse (a form of no true scotsman fallacy). Note especially that the person who talks about the curse "going dormant" (instead of being bogus) also works for an organization which profits from the additional attention it draws to the stone.