The elf herbalist gal you can srew gives you access to a shortcut to the quarry (from what I recall at least). And right after you go throught that door beside her and enter the quarry, to the left there's another wooden door which is locked.
How do I open it?
If it opens through naturall story progression and is unmissable, please say so.
If it doesn't , please tell me how to open it spoilerfree.

Natural story progression. You went with Iorveth, right? Don't really remember, but I think that door only opens when you side with Roche. Might be wrong on that (it's been awhile), but it's definitely one of those things that can't be missed if going in there is possible on your current playthrough.