Back in my rock-hound days I took several trips from Queens, NY to NJ for specimens. My trip to Franklin was mentioned above. I made a few to a trap rock quarry in Summit where we would spend the day swinging sledge hammers breaking up big rocks to find small pockets of crystals. The Sayreville clay pits were another destination where we would find concretions and on rare occasions, amber.
There was a lot going on in that area of NJ for rock-hounds back then. I imagine most of it has been built over with homes now.
Plenty of the iron-mining areas in NJ are undeveloped; much of it is on state land/in parks.
Northwestern NJ saw a brief boom a few years ago then it faded quickly as the people expected to flee westward from the high taxes in northeast NJ opted to leave the state altogether rather than live in the hills spending hours each day commuting to the east anyway. The property taxes started climbing there (and in central Jersey) as the urban sprawl spread; now they are looking at closing schools as the state depopulates.