It looks like galena ore. Galena is an ore of lead, and is found in ore bodies that may also contain iron, which will rust when exposed to an oxygen molecule. Hence, rust. It could also be slag from a smelting process of some sort, or even some other type of metal working. The specific gravity of gold is 15- 19, which means it is 15-19 times heavier than water. The specific gravity for lead (galena) is 7.4 -7.6. Galena can form in cubes, octahedrons, and rarely dodecahedrons. If you get a copy of the Audubon Society "Field Guide to North American Rocks and Minerals, there is a great deal of information about all of the minerals, metals, and gems on our continent. I didn't see photos of the specimen you found, but that isn't unusual. There is a description of each mineral, and will some practice, you will be surprised how quickly you can pick it up. Is there a local geologist you could show your specimen to?
My wife and I went to a tot-lot this morning and she beat me like a drum. We found the usual junk, a few clad coins and etc., but then she found a dollar coin. It was one of the new gold color ones. In the meantime I found two Rocks that sounded like coins below the wood chips and about 4 inches into the ground, I guess about 8 inches altogether. They were about 10 feet apart. I thought they were Iron pyrite or melted lead, but when I checked them they were very hard and heavy. They are non magnetic. They are not aluminum or lead and I don't know what to make of them. They both look alike and the picture is the larger one. They are deeply pitted on the surface and this one also shows a little structure on the end, like maybe a Crystal formation. Bill In Texas 