Google a "poultice" and the follow the instructions. You can do a search on here as stains like that are all over the place and most never come out.

You need to pull out your range to do this properly. The side edge of the counter, that is hidden by the range, was most likely never sealed.

Also most of the "white" quartzites aren't really quartzite, but some type of marble or dolomite. They might be hard like quartzite, but are usually porous like marble. These types of stones need to be sealed regularly.

Did you do any of the tests, discussed in the Geology threads written by karin_mt ,when you purchase the slabs?

There are about 4-5 quartzite horror threads here, but one had a paticular poultice of something like Dawn dishwashing detergent mixed with baking soda or something which produced results.