Why do dark stains appear on gold jewelry?

Well, a year ago, my mom got me a gold bracelet for my birthday. Like 2 months ago, i noticed that dark stains were beginning to appear in the links, as if it was dirty, or something, which is weird, because as you may know, gold is a very difficult material to stain. When my mom took it to the jeweler, he asked her if i was going through chemotherapy, which i'm not. He submerged it into a chemical and it was clean. A week ago, those marks were starting to appear again. I got into research (i've been following the case a little obsessively) and the only thing i found is that those marks appear when the wearer is nervous, stressed or under pressure. Well, i'm just a junior and the stress isn't high during classes, and i'm on summer vacations, so i'm not stressed or under pressure. Do you know what else could cause those dark stains to appear?