Check out STT's SB sealer (we used SB Deluxe). We used it on two types of honed travertine, and on a split-faced travertine mosaic, and it didn't darken any of those. We also used it on White Thassos marble, I'm pretty sure it's polished, and there was no change in that either. If you call the company, the owner (Stephen) is very knowledgeable, and very helpful. The sealer is practically odorless, and extremely easy to apply and then buff off (any excess). I have no affiliation with the company whatsoever, BTW, just a happy consumer! Here is a link that might be useful: STT sealers

Thank you. Since water does not soak in after 30 minutes I wondered if it will take a sealer. If they try and then wipe off the excess, I assume that will be okay. I have a granite that will not take a sealer so wondered about quartzite. I definitely don’t want sealer sitting on top.

"So the 3' limit for span of 3cm stone is not applicable here nor is the requirement for support of cantilevered stone since the stone isn't cantilevered" It certainly is. You've got a huge span which appears to be more than 8' which is unsupported. The Natural Stone Institute allows 3cm natural stone to cantilever 10" without additional support, not 15". I've seen stone like yours sag and when it does, it torques the stone. If you develop a crack starting at a sink corner heading to the unsupported edge, you'll know why. This isn't going to happen right away; it's going to take years. But hey, no worries, you've got a sign off from your fabricator, GC, and cabinetmaker.

Acetone removes sealer. Use it sparingly, and keep the windows open.