Found a heavy metal-like rock today

could be (Im nit dwelling on that but I would get it assayed at some school) Theywould probably do it for free.
We usually use a set of magnets with different strengths. A Cr/Fe oxide would still have a slight "tug"
A rarer mineral is Glaucodot but Are you sure tht you did the specific gravity correctly?

There are a number of locations for glaucodot in the US and many are in the East Coast. Im not sure where your area is roughly.

When I measured the specific gravity I did the following: Tied a dental floss loop around the mineral. filled a glass up with distilled water. Setup the scale in grams, and measured the mineral weight: 157 grams. Then I placed the water filled glass on the scale and zeroed it out. I suspended the mineral in the water making sure not to touch the bottom or sides of the glass, the measurement was 25 grams. 157/25=6.28. The second test the mineral came out at 158 grams, and remained 25 grams while suspended.

I am in Northeast Florida, but the driveway was composed of slag and lava rocks, so it could have been brought in from anywhere.

is the slag a metal process waste ? Ill look up some S gr's from metallurgical metals

It may have been, there were solid green pieces of slag in the mix.