Topic says all. Googled it, checked UESP and ESWiki. I've played Morrowind, Oblivion and Skyrim but this was also never mentioned. Never actually read Elder Scrolls books (Except the Book of Daedra and the Mythic Dawn Volumes). Anyone knows what souls gems are made of?

Don't think we know this, so far we found out for the 1st time that they can be found inside of the mineral (or whatever the heck that is?) called geode. Ofc I'd be interested to see more info on it as well, if someone knows....

the appearance of the greater soul gem in morrowind (a cluster of crystals) would seem to suggest a mineral origin, as does the "geode veins" in Skyrim, but I don't think more than that is ever revealed. I'm racking my brain for anything related, but its been quite a while since I last brushed up on lore, and the only things that come to mind related to soul gems are

1) the passage in a book (the maddness of pelgius I think?) where a soul gem is described as having a sort of spectral wolf visible inside it,

2) another possible origin of soul gems being meteoric glass like welkend and varla stones (magic from the sky I think)

3) a passage in a book (Souls black and white I think) adivising caution when handling soul gems to prevent them from absourbing part of the handler's soul

that's all I can think of, but I'll do some digging, interesting subject, this

On earth, geodes are hollow formations found in certain types of rocks that have crystallized mineral deposits inside them, most commonly some form of quartz. The six-sided shapes of the soul gem models in the game would be consistent with quartz crystals on earth as well.

Take it for what it's worth.

How do you get to the center of a Soul Gem? The world may never know.

Obviously they're based off of real world http://www.lovehelpandserveall.com/crystals%20pics.jpg. They took the crazy idea from real life that crystal can hold energy/information and use it for magical purposes in TES.

Notice how there's different colored soul gems like different colored crystals in real life that are formed from different minerals/materials....that's likely the reason why they don't specify what they're made of as there's different types.

http://www.uesp.net/wiki/Lore:Liminal_Bridges

They're obviously just an element unique to Nirn. Naturally occurring, not manufactured.

Can't you mine soul gems in blackreach? Could be mistaken

You can yes.

nice research, I would give a cookie but you'll have to settle for some sweet ice cream for a job well done