Demystifying and questioning the very notion of authenticity, Jason Kohn’s informative and oddly riveting, diamond-documentary “Nothing Lasts Forever” is ostensibly about the oft-antagonistic relationship between natural and synthetic diamonds. Yet, diamonds are an in-road as Kohn explores the commodification of such abstractions as love and desire, questioning how exactly a shiny rock — one that isn’t even that rare — became a physical manifestation of commitment.