Found in Northern New Jersey. Very light weight, not magnetic, found in an old colonial town right alongside a river bank. Roundish shape, slightly bigger than a baseball. Was picked up by metal detector, not my eyes.
I think you have graphite. Very light stones and you can write with it. I find them in lake Huron and some are huge. Yes my metal detector picks them up also.
1st - I noticed this was your very first post - so, Welcome Aboard BenFrankOG! You didn't list your state (or country) in your profile. So, you might consider jumping over to Sub-Forum: Select Your Area.... for information (i.e., clubs, hunts, finds, legends, maps, etc.) directly related to your state (or country).
2nd - You might consider posting your pictures on WHAT IS IT? for more exposure...
I'm going to take a long shot here, but the only thing I've ever seen that looks like this is heavy crude oil that has picked up sand and debris and bound them together. There's a name for that but I can't recall it. Does it float?