Manual has no word on what I'm about to write here.
Foraging skill is raised by lookigfor plants only. It's rise is not affected by gathering stones, grass or scrap on junkpiles. But foraging affects the quality of everything you will find in nature:
1. Stones - Think quality of chalk you find. Quality of bricks you will make, possible quality and durability of your walls and pillars. Also quality of your foraging tools. (even at lvl 80 you will not get too many very high quality stones)
2. Grass - Quality of your grass armor and baskets is directly related to foraging. If you have poor foraging skills you will never be able to make very high quality containers.
3. Branches and twigs - same as with stones. Used in toolcrafting and in architecture.
4. Scrap - I have yet to see relation between quality of scrap piles as I was able to get master quality stuff from poor scrap piles (like poor leather crap would give me master leather of some sort). This part needs extra testing.
Foraging is extremly easy to level. At around level 25 you need less then 30 attempts to gain a level. Up until level 65 you need around 30-35. From 65 to 80 it starts growing rapidly but still very easy levels, at 75 you need around 55 attempts. It took me 73 attempts to go from 79 to 80. This is where I'm at now.
As soon as you reach level 70 you will be getting very high quality stones every now and then. At 80 most are very high quality (beware of stacking as it messes up your quality).
Same as with scavanging at over 60 you start getting plants you have never seen before. Some are edible but apart from that there is no effects I have noticed, I guess we need to wait for cooking to be implemented. At over 75 all plants you find become very high quality, no exceptions. You still find sierra yampah and purple milkweed but it's rarer then usual.
My guess is player at level 100 foraging and toolcrafting never has to worry about master quality tools again.
Enjoy if you didn't know how important foraging was. Not even guides were able to tell me if it will be affected, it was all done by testing in the past 48 hours.