BEST POLYMER CLAY ¿?

For jewelry or anything else you want to be strong even in thin areas since there will be later stress, just avoid the brands/lines of polymer clay that'll be brittle after baking in any thin or projecting areas with later stress (Sculpey III, Bake Shop, probably regular Craftsmart, no-name brands, Super Sculpey Original, and especially plain Original Sculpey...and I've lately heard from one person that even Super Sculpey-Medium and Super Sculpey-Firm can be that way but can't confirm).

Those and other "too soft" polymer clays also won't be able to achieve and hold as crisp fine-detail as the firmer ones, can be sticky or get fingerprints, etc.

I usually recommend Premo unless the person really wants one of the 2 firmest brands/lines (Kato Polyclay and Fimo Professional for the colored ones). Premo is medium firmness, easily available, comes in lots of colors including special colors, handles well, etc.

You can read more about some of the characteristics of other brands/lines (both colored and neutral-colored) in my previous comments here, if interested:

... the colored polymer clays: https://www.reddit.com/r/polymerclay/comments/9uk8c2/i_need_suggestions_on_good_clay_brands/e96dxdr
... the neutral-colored polymer clays (sold mostly in bulk for painting and/or as skin colors): https://www.reddit.com/r/crafts/comments/9vs0ow/airdry_polymer_clay_and_acrylic_paint/e9fxeyn https://www.reddit.com/r/somethingimade/comments/760rhv/orca_whale_super_sculpey_and_acrylic_paint/doc0qwe

Btw, if you're thinking of selling only the newly-trendy dangling cutouts type of polymer clay earrings, you'll be competing against a load of other clayers doing the same thing. If you'd be any of the other zillion kinds of polymer clay earrings though, or the multiple-dandling-cutouts type would only be one type you're offering, that would be a more viable business.

You might also want to ask questions about polymer clay in r/polymerclay.

And my polymer clay encyclopedia site has loads of info about just about every aspect of working with polymer clay, if interested:
http://glassattic.com/polymer/contents.htm (scroll all the way down that page to see all the topics covered)