Were you near a place that had railroad activity ? I found a similar lump a few years back that was melted babbitt metal. In the old days instead of replaceable bearing inserts babbitt metal was used as a bearing such as axles among other things. When it get's worn out they would melt out the old babbitt and melt & pour in new material. Where the old metal hits the rough ground and cools it often looks exactly like your picture. It was also used in automotive applications as well. It's usually an alloy of lead, tin, and possibly some other metals. I can almost guarantee this is what you have. Here's a large chunk I found near a railroad repair depot. Hope this helps