Gold anodes

Can someone tell me a little about small Gold anodes. I bought a
stock of misc. anodes from ebay a while back. I am trying to make
sure which ones are brass and which are Gold. I put Nitric acid on
them. I for sure can tell which are brass, but I have 3 that acts
differently. They had a thin film of plastic on one side. I peeled
this off, put Nitric on each side, one side did not fizzle and turn
green like the brass, but after the nitric was on for a few
seconds, I could tell that it started to get active, then when I
rinsed it off, it was silver looking where the nitric had been. The
other side of the 3 anodes is a different shade of color and acts
similar to the brass. Do gold anodes come just a thin layer on one
side? The ones for making jewelry? How is the best way to tell if
they are gold? Clint