The rock is very dark (black) with metallic colored streaks/small patches.
It weighs 350 grams.
I’ve measured its volume (via imprecise water displacement) at between 50-55 cm3^3. So its density is probably between 6 and 7 g/cm^3.
There are a couple faces that appear to have been formed by pieces of the rock breaking off relatively recently in its history. The second picture shows one of those. These faces contain many tiny metallic/sparkly pieces and are lighter in color (grey, not black).
Refrigerator magnets do not stick to it.
I found it over ten years ago in western Pennsylvania, specifically a park in New Kensington. It was sitting on grass next to disturbed soil. I remember thinking that either someone had dug it out and left it or it was kicked up by a lawn mower.
Alcoa had facilities in New Kensington going back over a hundred years. Between that and the possibility of industrial byproducts from other random ventures being scattered wherever, I’ve always assumed that something related to that is the most likely source of this rock.
Sounds like it will have to be an ore of tin, lead, zinc or some such. My first guess from the description was going to be granite, which often feels heavy for its size, but assuming the OPs measurements are correct, this sample is over twice the density of granite.
Tin or lead ore seem to have the right density but I didn’t find images that look particularly close to this rock yet. Some pictures of zinc look pretty close but it seems like zinc ore is too light. And the mass I’ve measured is definitely accurate while the volume is pretty close to accurate: it’s not that much larger than a golf ball.
Sadly, I think I got rid of all my strong magnets when my son was born. I’ll try to get my hands on one to definitely rule out iron and such.
The industrial byproduct guess is probably a good one. I don’t know what aluminum slag usually looks like, but other slags I’m familiar with tend to be dense amorphous things. Towns that produced a lot of slag also often liked to use it for paving and landscaping and such, so finding it in a park wouldn’t be too unusual.
Slag is a good prospect. Western Pennsylvania has mountains of slag available. But it does seem pretty dense, magnetite has a density of around 5.2 gm/cm3, iron is close to 8, so this is pretty dense stuff.