Each slab should be tested for porosity. It is quite simple , just place a palmsized or smaller puddle of water. Let it sit for 10 minutes or so. Then wipe away with a paper towel. If you are left with a dark mark you just confirmed your material is porous. The longer the spot takes to dry out gives you an idea just how porous the material is. Then it is your decision to hire someone or do it yourself. In your case Heidia you need to remove the stains first. Something you should have your fabricator take care of for you. He should reseal the surfaces. You should be aware of what he uses and should leave you some. You should do the water test 24 hours or more after sealing. The reason is sealer takes time to cure and you will get a truer result after a day or more. If you still get a wet spot from water it needs additional applications. I use the word applications because when we seal one application will usually be more than one coat. Test a day or so after each application for results. While I consider sealing a professional task I think it can be done better by a homeowner. The reasons are quite obvious as the many posts written on garden web regarding insufficient sealing of new stone. Homeowners will always be there to test and if needed apply additional applications. Like ott2 stated the sealer could be seen penetrating the surface. When the surface dries back to normal repeat the process. Again when I seal a countertop that is porous I try to get as much sealer into the surface as possible. Again one application to me can be many coats but still one application. Then test after the cure. You will notice a major change in the absorption rate. Very important point-if you test your unsealed stone and it isn't porous and absorbed nothing do you need to seal? Chances are that your stone is very dense and will not stain. My opinion is it doesn't need sealer. However we are flooded with sealer marketing and hype so for some folks they wont be able to sleep if their non porous surfaces aren't sealed. If this is the case just make sure that all the sealer residue is removed from the surface of the stone. If not it is possible to encounter other issues caused by the sealer residue. Also I am always asked about how often people should seal. Easy answer-use a neutral PH cleaner that will clean your surface but not degrade the sealer. The water test will always tell you when surfaces become porous. Sealers will last if maintained properly for years. Most sealer companies market sustained sealing capabilities but then somehow have folks thinking they need to seal every six months to a year. Always keep in mind the actual performances of an impregnating sealer. "to temporarily inhibit the intrusion of staining agents from the surface of the stone" End of list of performances of an impregnating sealer. One other important note-use a sealer that will protect against oil based and water based stains. Hope this helps.

"Stone is porous and unfortunately hold undesirable attributes because of this." @Jason Martell resurrected a 5-year-old thread to add an ignorant statement. For anyone reading this--now that he has brought it up to the top from 5 years ago--please note that while some stones used for counters are porous, there are so many that are not. Many stone counters, both of granites and quartzites, need no sealant at all, ever, due to their low absorption rate (i.e. very low porosity). Some stones have such low absorption rates that the use of sealant mars their appearance; since the stone is so dense, the sealant cannot be absorbed so it sits on top, and causes the counters to look hazed and streaky. There are a number of posts on this forum from people who sealed counters of a more dense stone, and got that result. Quartzite, like granite, has different varieties that vary in porosity. Taj Mahal is one of the well-known quartzites that is not porous and does not need to be sealed. However, the OP's quartzite, Calacatta Gold, is not as dense and will need to be sealed. A Deep Dive into the Properties of Quartzite

kris G: I hear you. I continue to learn to live with to try and enjoy my stone. Even do my best to accept the stains (husband just recently left something clear but oil based on and stained-I used acetone and it did lift some. But eek!! ). We now use silicone mats under everything. And I only plate meals at the table now. But, even as careful as I am, and quick as I am to clean if something drips, it still can stain/leave a Mark (ie transferring raw bacon to oven pan, liquid dripped, I immediately wiped and it was set). I know no one would notice but it’s hard that I do. I try to no longer be hyper vigilant about it, but, my kitchen has never been clean like this! Haha. Sorry and hopefully you can find a good sealant and some grace

Mr & Mrs Perfectionistos Do you like the super shiny finish of the Quartzite? I am looking at the Cristallo. The color is perfect,but it's shines like glass. I am not sure if it will be too fancy for my space? Has anyone installed leathered cristallo....is it a smooth or pitted texture and would it ruin the look? Below are pictures of my kitchen with tons of different samples, the Cristallo slab, testing of Cristallo sample and a piece cardboard on island to show new size(1'bigger than current island). Any thoughts?