Yep, copper. But not usually filings. It's a chemical reaction that happens in just the right circumstances, usually involving cupric oxide, heat, and oxygen. The copper in the cupric oxide "precipitates" out -- forms tiny gold-like crystals. Take it too far and you might get oxblood.
The gold aventurine (only marble collectors call it "lutz") in antique German swirls (like that nice lutzy onionskin ^^ up there) was not usually made in Germany, but was imported from Venice. Still is.