On Nirn (or at the very least in Tamriel), soul gems seem to be extremely common as they're found in almost any shop anywhere.

As I remember, the only source of new soul gems is the geode mining nodes in Blackreach.

Is this (and similar areas across Nirn) the sole source of soul gems in the world?

How come there are so many in circulation if they're only found in such remote locations (and seeing how they're destroyed upon use)?

Official quotes and sources much appreciated

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3 Answers

Soul Gems where originally created by Molag Bal (Full text). Whether he discovered how to shape the crystal or created it is not clear.

Beside that, it seems that the consensus is that they are mined from the ground around Blackreach and Solstheim, where magical ores (Aethernium, Ebony and Heart stone) are found. Maybe they are also found in other parts of the world and imported in Skyrim. Keep in mind that Nirn is built from the bodies of gods, so unusual crystal formations and extensive caverns can be expected.

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Well, I can answer part of your question. Pretty much, soul gems are crystallized formations of energy, they form in oblivion, they then bleed through into the physical world, through weaker spots in reality, the weakest places are in blackreach. I do not, however know how the shopkeepers are somehow able to have access to them so readily. This probably didn't help, but I hope it did.

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Shopkeepers have plenty of them because the Player needs plenty of them. The story of the gems exists ro make them sound cool and mysterious.

It might be completely wrong from an logical point but logical when you consider that the deleopers want you to have as much fun with them as possible