Firstly, jewelry would still be motivated by scarcity, even without a production cost or time cost. Scarcity still exists for seemingly "free" stuff. Our bodies only have so much available space to display it, and there is also a social cost. You need to think about what jewelry represents; it is a social differentiator tied deeply into mating and power. Think how men and women use "free stuff" today to fulfill those desires.
A hairstyle, for example, can be produced "free" of charge in your bathroom with a pair of scissors, but you have only one head of hair, and there is a social risk of making an ugly haircut. So due to the social risk, and desire to differentiate ourselves in the mating process, scarcity will still exist.
So to answer your question:
- It will be deeply tied into the mating desire
- It will be anything a person can use to differentiate themselves, which would naturally create scarcity.
I would expect to see risky, over the top, jewelry with a short trend life. Since production costs would be close to zero, people would have to try harder to differentiate themselves from the crowd. The low production cost would keep trends on a shorter life cycle. Popular trends could be adopted quicker, which would lead to a quicker peak.