One of the survivors finds a glass ring (something like trinitite) among the post-nuclear-blast rubble of Saks Fifth Avenue[3] in New York and sees visions of the future (or something) through it.

[1] https://en.wikipedia.org/wiki/Swan_Song_(McCammon_novel) [2] https://en.wikipedia.org/wiki/Bram_Stoker_Award#External_lin... [3] my memories of this book may be embellishing this a bit....