QUESTIONHello, I have white/gray honed marble countertops in my kitchen. They were installed three years ago and tonight I noticed a 2 x 6" darkening at the edge of the sink. I can send a picture if you like. Do you know what this is and what I can do? Thank you in advance for any advice you can provide.
ANSWERANSWER - Marble isn't the safest material to use on a kitchen countertop because it is primarily comprised of calcium carbonate that is chemical sensitive and has a tendency to scratch. Having a honed surface does make it resistant to scratching and chemical etching might be less noticeable. Although marble is not soft as some non-knowledgeable people like to say about it. Marble is very dense and impervious as it has an absorption of 0.02% of less.
Without doing the forensics on the dark edge of the marble that is adjacent to the kitchen sink, I would guess you might have some microbial growth migrating into the marble. That transition joint from the sink to the edge of the marble should have been filled with an ASTM C920 sealant (caulking) such as 100% silicone or polyurethane sealant. The sealant not only mitigates expected movement stress at that transition, but it makes the joint water tight. So if the edge of the marble is subjected to prolong moisture and organic contaminates then microbial growth can occur.
The stain could be caused by other things, but you would have to do the forensic to determine what it is and the cost might not make that practical.
You can use poultices to try to remove the stain. You would need to remove the grout at the edge of the stone so the poultice is in contact with the marble face and edge. There are many types of poultices. It would be best to hire a processional stone restoration company to try to remove the stain.