Thanks for the helpful info!, I can write with these rocks, they leave dark trails when enough friction is added to them.
I'll see if it's magnetic in the morning, I'm too tired right now to do anything besides type.
If you can write with them, it sounds it could be a kind of coal; higher grade coals (i.e. anthracite and obviously graphite) will leave a trail when rubbed across paper. An anthracite would ignite relatively easily, while a graphite usually does not. The law of superposition would lead me to believe that if you found these on top of or in the A horizon of the soil, they are relatively new, so they could possibly be either slag or mined coal that just got dropped at some point, as the coal could have been used to fire the pig iron forges to make the intermediary product between a mined iron ore and steel.
Anthracitic coal will have a hardness of 2.75-3 on the Mohs scale, meaning that you should easily be able to scratch it with a fingernail or a copper coin i.e. a penny. Graphite would be even softer, you could probably scratch graphite with a fingernail. Slag would be harder than anthracite, meaning you'd need a steel file or knife to scratch it.
Here's a Mohs chart for reference.
