Your nugget of combined silver and copper is most likely what is
popularly called a “half breed” Some of these specimens occurred in
very large sizes. They were typically found during the mining of the
Keewanai Peninsula copper deposits of Lake Michigan. I once had one
which weighed about two pounds and was worth fifteen hundred dollars
! Unfortunately, because of their value, many have been assembled and
/or “manufactured” Ron at Mills Gem, Los Osos, CA.