And last but certainly not least on the Functional Jewels that I have yet to offer the public....  moto gas caps.
 
This style being a Cam Lock (Bayonet style) Vented gas cap. The cap itself is chrome plated steel, and machined by a motorcycle parts distributor to ensure quality part. But the adornments upon it, are my silversmithing creation.
 
For this piece I have a made a sterling silver diamond pendant, a central diamond of Python shed rests under glass, a thick gauged rope wire edge around it, with high contrast patina to highlight it all. 
 
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Slight discoloration on the cap itself is natural, and unavoidable as it was heated, as its an impure/not fine metal. It will look congruent to what you experience on parts of your bike that get heated up with active use.