Historically, stranded copper hook up wire was tinned. Obviously, wire intended for operation above the melting point of tin would not use tinned strands. A quick tour of web sites supplying high temperature appliance wire shows nickel plated copper strand construction is common, so that would be my remote guess. Look very carefully, with magnification, at the cut wire ends. Some copper color should be visible.

Pure silver wire has lower resistance than pure copper wire, but this is generally only important for very low voltage functions where wire resistance has a significant effect on electrical efficiency, such as wiring together solar cells.