I have a problem with obsidian going obsolete.
I have at least 10+, I counted 4 around 3 cities, all with quarries, but is says I have Zero obsidian resources available/in cities.
I feel this may have happened around the time of Architecture and the obsoleting of Stone cutters.
I still have stone and marble, and they are being used by the masons guild.
Spoiler :attach photo on no obsidian
If its just me, its my game, as I've noticed a few cities don't have a working foundry, with pop's of 15+ but with 2+ upset citizens, maybe that's it?? and a lot of inland cities seem to have a fishers hut, with only a river or lake and no fish, crabs, shrimp,whale.
Obsidian goes obsolete with Metallurgy tech. If you have foundries, then you have Metallurgy. The only units that can be built with Obsidian are Ancient Era or the Classical Era Heavy Horseman. Once those units go obsolete, there isn't a need for the resource.