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There is item (it is high ranking decoration of one country) that is supposed to be made from gold-plated silver with some (white) enamel. Here is the picture:

Gold plated silver decoration

My question is - why there is this rainbow color, especially on the rays (especially on the upper-right rays) in the picture? What this rainbow means?

I have different ideas. E.g. that the gold has been removed by some fire and that left these marks. But some ordinary people said me, that temperature of gold removal or fire would be terminal for the silver which has very low melting point.

So, maybe this rainbow is the mark, that gold has been removed by some chemicals (king water?).

Or maybe (in the worst case) the rainbow color is the indication that this is base-metal fake that has been made by some copying process (I don't know, litography maybe?) that left such marks?

I am afraid that such rainbow marks can be indication of copying, of item being fake?

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    $\begingroup$ I see that on screws and drills. So definitely doesn't have to be gold. $\endgroup$
    – DKNguyen
    Commented Nov 17, 2021 at 19:58

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the metal surface has a thin layer of oxide on it which causes interference fringes in color to appear. this same mechanism puts brightly colored fringes on the chrome-plating on motorcycle exhaust pipes.

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    $\begingroup$ The question is, will such an oxide layer form on gold? I am not a metalurgist, but I strongly suspect that the answer is "no." $\endgroup$
    – Solomon Slow
    Commented Nov 17, 2021 at 19:49
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    $\begingroup$ @solomonslow, the metals are probably not pure. the oxide looks like silver oxide. $\endgroup$
    – niels nielsen
    Commented Nov 17, 2021 at 19:55

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