Great game. Spent 4 hours just foraging the other night to build up some ingredients for poison making for my stealth archer. Enjoyed every minute of those hours.
Surprisingly though I could not find graqes, lichen, or pearls. Which would all seem easy to find and level independant.

Others for comment...
powdered mammoth tusk, can this be made from a regular mammoth tusk? Same with charred skeever hide, kill a skeever with a certain flame spell?
Jarrin Root, anyone?

Are you sure Jarrin root is a regular ingredient in the sense that you can find them? I've only seen it in a dark brotherhood quest, never on any vendors or in the wild.

Actually is lichen even foragable, or is it a mob drop?

Jarrin can be used as a regular ingredient. I have not encountered one though. Maybe it is semi-unique?

The jazaby graqes are difficult to find because they blend in - if you want to know where they are, send me a private message and I'll let you know. No spoilers on this board will I post.