Apart from bagginsbill's metal shader techniques, which are awesome, the key to iridescence is the HSV node. Since the hue input values revolve around the ROYGBV circle, multiplying the hue by a large integer and feeding it from the Fresnel node, which addresses angle of incidence at the sphere's surface, gives an adjustable band of rainbow hues.

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Verbosity: Profusely promulgating Graham's number epics of complete and utter verbiage by the metric monkey barrel.