Machoke and Machamp are odd. According to this page, their trunks are just a pattern on their bodies rather than actual clothes. Meanwhile the Pokédex has said that their belts are removable. While that answers the question of their trunks, it is still up in the air as to whether their belts are artificial or if they are some kind of extremely complex organic feature that can be shed, and potentially reworn, at will.

(On a similar note, Throh and Sawk’s belts are fashioned from vines according to the Pokédex, although I suspect that their gi are actual skin/hide.)

So Zoroark could go either way. The gem could be organic, but there is plenty of room to think of it as an accessory, much like how Spoink gets their psy-enhancing pearls from Clamperl.