I’ve had the same issues with our quartzite since having it installed two years ago. Darkening around the edges that poultice or any other cleanser would not remove.
as someone else posted here, it is silicone staining from when it was cut at the shop. I went with a recommendation of someone on either this thread or another of the dozen other I was looking at to remove the staining and it worked.
I purchased Lithofin Oil-Ex. I wiped the whole stained area down with acetone, placed the Oil-Ex everywhere I was seeing the silicone stains, covered it in plastic wrap for a day, removed the wrap until the oil-ex dried out, then wiped it into the trash. The liquid from the gel absorbed into the stone but hours later it dried and the stains were gone. Some spots needed a second application but it’s significantly better than before.
I wasn’t smart enough to do before/after photos but I’m very pleased. I’m in the process of resealing everything now. I hope this works for others. The oil-ex is expensive (about $40 per tube, I needed two) but it’s worth it!
Q