I nearly always bring a scroll of magic mapping for this quest because there often is a small separate room somewhere on the map where the last couple dark ores are found. I find it very tedious and food/health intensive to blindly search for the room. I mostly only skip the scroll of magic mapping when I'm very overpowered or have strong sustainability.
I found more than 40 on my first runs but it wasn't easy..., then I tried to fight that big crystal, consumed all my food (and horn of plenty) and nearly died. I like how challenging it is to be honest!