First, it is not a fixed question whether jewelry can be covered by copyright at all. Copyright extends, among the rest, to “pictorial, graphic, and sculptural works.” It is arguable whether jewelry can be classed as sculpture. Some can be, others not.
“A ‘work of visual art’ is—
(1) a painting, drawing, print, or sculpture, existing in a single copy, in a limited edition of 200 copies or fewer that are signed and consecutively numbered by the author, or, in the case of a sculpture, in multiple cast, carved, or fabricated sculptures of 200 or fewer that are consecutively numbered by the author and bear the signature or other identifying mark of the author…”
Most commercially designed objects can be protected under patent law, which is different from copyright law. One can apply for a design patent, but this covers mechanisms rather than the artistic aspect of the piece.
Copyright does not cover ideas.
“In no case does copyright protection for an original work of authorship extend to any idea, procedure, process, system, method of operation, concept, principle, or discovery, regardless of the form in which it is described, explained, illustrated, or embodied in such work.”
Copyright only covers the specific expression of the idea, not the underlying idea itself.
So if Moby Dick were still under copyright anyone could write a novel about a bonkers sea captain chasing a giant whale, as the idea of the novel is specifically excluded from copyright.
These parameters are specific to US copyright law. International copyrights are covered by the Berne Convention, to which the US is a signatory but which may have slightly different definitions and strictures.
But, as was said above, ethical behavior is different from legal behavior.