How to clean marks of a black granite hearth?

[VivaLeBeaver]

Just had a new hearth fitted and there are a couple of fairly large smears/streaks on it. I just assumed it was a bit damp after it had been fitted and it would dry. Te smear is the same colour as the black granite, just a bit darker so looks like a smear as if someone has wiped a sweaty hand on it.

But its not drying and its not coming off. I've tried with a microfibre cloth and some fairy liquid.

Anything else I can try before ringing the company up and complaining?

This might sound strange but WD40 is brilliant. When we had a woodburner installed last year on a granite harth that's what the installer recommended and it works brilliantly but does smell for a bit afterwards.

Thanks, will try that now.