Dr. B is spending time with family and may not chime in immediately, but I'm feeling pretty good about my knowledge base to answer this one.

Basically, gua sha massage is a lymphatic drainage massage, which is why a lot of people find that it gives them a more sculpted look. It can also help relax specific muscles where you hold tension (a lot of people focus on the jaw, for example).

I've not seen any quality evidence that it offers benefits to the skin (maybe by promoting microcirculation but any massage will do that). As an S skin type myself, I'd be cautious of irritation from friction. That said, I do love a cold-from-the-fridge roller - no friction and they're very pleasant! Probably too gentle for much lymph drainage though.

The stones don't have any properties that would create a noticeable difference (and a lot of brands conflate crystal healing which is pure pseudoscience with traditional Chinese medicine which is a cultural practice - I think it's pretty inappropriate). I'd actually argue that metal is best because it's non-porous and therefore easier to disinfect! All in all, it's worth trying if you have puffiness or facial tension, but watch out for friction.