Mystery Rock Found in Waldron Shale Pile

This rock was found in a pile of Silurian Period Waldron Shale next to a busy parking lot.  One side of it has a black layer that attracts a magnet.  [I sometimes test things found with rare earth magnets after watching the Science Channel show Meteorite Men.] The rest of the rock does not appear to be limestone.  As an experiment, another rock with a horn coral in it known to be Waldron Shale was exposed to vinegar.  It began to fizz.  Vinegar was then applied to this mystery rock and nothing happened.  In another test, it can be scratched with a steel knife. [Update: I wet sanded the cut side of the rock and it left just a black residue on the paper.  Also sanded during the session was sphalerite (zinc sulfide) which left a cream colored residue and smelled like rotten eggs while being sanded (sulfur?) and a small piece of metallic rock (1 g) that left brown residue while sanded.  So I can conclude the mystery rock probably does not contain sulfur.]

It dimensions are roughly 2.5 cm x 2.1 cm x 2.3 cm with a weight of 38.5 g.  The side with the black coating is smooth but the side opposite that is rough (see next picture).  The other sides are basically flat but coarse like a 200 grit sand paper. On the surface of a few sides are several small pot marks with diameters 1-3 mm.

 Closer view of side that attracts magnets.

The side shown on the left of the picture was cut with a diamond saw. It is almost black after being cut with small holes the size of grains of sand.  The water coming of the saw blade turned darker when cutting, reminding me of cutting black New Albany Shale but without the sulfur smell.

Some one must of discarded it into the pile or it came off a piece of quarry machinery?  I assume with the coating and bubble cavities it was man-made.