How can I remove a stubborn stain on my new granite countertop that the installers were unable to fix?

Do you have any suggestions on how to get rid of a stain on my new granite countertop? I noticed a darker color along the seam a few days after it was installed. I tried washing it with dish soap, but it didn’t work. The fabricator and installers have tried to fix it, but the stain is still there. Any ideas on how I can tackle this problem on my own?