So only 6 obsidian are available in the game (and is that all that's needed to do all upgrades)?
Glad you found the post.
What I listed is all occurrences of
$obsidian += in the game's code (unless my script failed, but I think it didn't).
As you can see you should get two obsidian units from quests in the tavern, one (iirc) from a chest in the maze, one in a wooden crate, one from Skye upon meeting, two from a skull and two from a headless statue.
So there's more than six units.
As far as I remember, they cover all upgrades but beware spending all obsidian before you fully upgrade the tavern, because then you lock yourself out from quest 7's reward, which is another obsidian.