I've played Morrowind, Oblivion, and now Skyrim. I find soul gems all over the place, and all sorts of merchants have them for sale. They must be a pretty major industry somewhere in Tamriel. Yet, I've never found a soul gem mine, nor heard of any sort of arcane apparatus that manufactures soul gems. Has there ever been any Lore about where all these soul gems come from?

If there's crafting, why can't I craft soul gems, or mine them? Do they only come from one mine somewhere in the Summerset Isles, or Hammerfell, or something, hence why I've not seen any source for them in Morrowind, Cyrodiil, or Skyrim?

Blackreach has Geode Veins, which contain soul gems. These veins are no doubt all over Tamriel.

That's my guess.

They occur naturally within Nirn. You dig them up

Blackreach lol. But it makes me wonder how the whole world gets access to these veins when there's nobody down there except Falmer... Those sneaky mining blind business men...