Iodine was spilled on my cultured marblel/onyx sink. I've tried all types of cleaners; nothing has worked so far.

Doubt you will get Iodine stains out. You might try bleach on a rag and let it soak on it.

I found this,

Removing Iodine Stains From Marble

Stone Surfaces - How to Remove Iodine Stains | HowStuffWorks

Stain Removal | The Marble Man

Quote:

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What we can do to remove your stains
The most effective and common method used by a stone care professional to remove a stain from natural stone is to apply a poultice

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And since I like videos

how to remove iodine stains from marble - Bing video

This is a series of videos but they all say make a poultice specific to the stain. Let me know if this helps.