How do I get rid of a gray discoloring on my black marble bathroom ctr

Harsh cleaning chemicals can cause this problem along with acid drinks like orange juice. Etching can be repaired by using an etching repair kit for marble (Amazon sells one). Use only a cleaner for granite and marble . If it's cultured marble the dulling may be that water has gotten under the sealer. You'll need a professional for that.

Using a solution of warm water and dish soap, thoroughly clean the surface of the marble. Use the rough side of a kitchen sponge to gently scrub the surface. Dry the marble completely with a clean rag.

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Fill a spray bottle with hydrogen peroxide...see the rest at http://homeguides.sfgate.com/clean-discolored-marble-tabletop-90974.html

I read that bleach wipes will cause marble to discolor, so be careful not to use those on your marble tops.

https://www.youtube.com/watch?v=b3xyV1sxZnQ