This nodule is reported to be a result of electrowinning of copper, a process in which electricity is used to reduce copper oxide to copper metal, as a means of extracting the metal from its ore. It's a lot like electroplating, the only difference is that in electroplating your goal is to put copper onto something, while in electrowinning the goal is to get the copper in the first place.
This nodule has almost completely settled the debate about whether my other copper nodules are natural or man made. I had thought that because they showed no visible point where an electrode could have been attached, they couldn't be electrolytic in origin. But this one also has no visible point of attachment, and I have no reason to doubt the claim of its origin (unlike the other nodules, whose claim of origin is unconvincing). So apparently there is an electrolytic process that does not result in a flat side.
Source: eBay seller fullhouse2b3g
Contributor: Theodore Gray
Acquired: 27 April, 2003