> The hardest part might be putting it all together in a way that can be trivially decoded by human descendants, even if they don't speak our language or share our subspecies.
Though other crystals should be highly stable. Quartz seems geologically and chemically stable, plain old glass ain't bad. Something reasonably cheap is probably preferable, both from the cost basis (an expensive-to-create archive is a challenge) and the repurposing challenge (a diamond-etched bibliographic archive might have other appeals to those who chance across it).
Even parchment proved sufficiently valuable that works were often repurposed (and lost) through palimpsests.
> The hardest part might be putting it all together in a way that can be trivially decoded by human descendants, even if they don't speak our language or share our subspecies.