My kitchen counter top is made of QUARTZ, and I have a small area that the shine is dull. Can the surface be restored to make the shine uniform as it was?
Do not use sealer on engineered stone, it has to be mechanically re-polished if it's lost its luster due to abrasion. If you're lucky it's just dirty.
Of course, you cannot apply a sealer to Silestone quartz countertops, so it's just a matter of cleaning it. –countertopspecialty.com
Do not use oil-based soaps for this. Dulling of the shine occurs because of residue left behind on the countertop. Oil-based soaps are often used on porous surfaces because they are designed to soak in, not to be completely removed. Standard dish soap is best. [...]
To remove cooking film from the quartz, spray the space with window cleaner. [...]
Avoid Using More Polish. Quartz countertops are polished within the manufacturing company, using a sealing surface polish. Further polish added to the top will only create excess buildup which will further the dull look.
–How to Polish a Quartz Countertop to Make it Shine, doityourself.com