I have specified this before and it did stain in one area. Soft Scub with bleach removed the stain.

Yes. Things like juice from, for example, frozen wild blueberries will leave a mark, but it comes right up with normal house cleaning products like the one mentioned above. We have light gray quartz at home (installed 5+ years ago), and we absolutely love it. It still looks brand new.

We recently installed white quartz. Everything stained it--even things I would not normally think of as capable of staining (like the backing of a drying mat would leave an imprint). Soft Scrub removed some but not all, so I ended up leaving it for a bit--and it bleached out a permanent stain... After this huge disappointment, I went ahead and had it sealed to keep the rest from staining (and subsequently getting bleached). And it totally solved all of our problems. I wish i had done it sooner, as my installer recommended. But I thought i was smarter because I googled quartz extensively and found that you are not supposed to seal it--well, the installer was right. Despite all manufactures claims, it does stain, and since it stains--it means its porous, and precisely for this reason, the sealer works and is really quite necessary. I think it is a marketing ploy to tell people "it does not need sealing"-- may be it's not as porous as granite, but it still benefits from sealing/

I have white Quartz and fruit or anything red stains it easily. We have not sealed it per installers instructions. I was afraid to use bleach but nothing besides oxy clean was able to get the stains out, which had seemed to have soaked in. Now I whip out the oxy clean whenever we need it, it works well.