Alex, of course now I'm poking a bit of fun at the silliness in the audiophile industry when charlatans attempt to peddle pieces of connecting wire at outrageous prices.
What you say about the jacket covering the wire itself can be true on rare occasions. Ordinary lamp-cord costing about 20 cents a foot, which I've had around for about 20 years, doesn't have that problem. However a poorly formulated PVC(polyvinyl chloride)jacket or one subjected to extremely high temperatures can experience a PVC breakdown, releasing the chlorine to combine with the surface of the copper, forming the greenish copper chloride. The oxidation reaction involving oxygen creates cuprous and cupric oxides which are an increasingly dark brown to almost black(e.g., old pennies).
