Copyrighting an original piece of jewelry...

First, copyrights don't protect products or ideas, they protect expressions of ideas. What a copyright will protect is the decorative part of each design that you copyright.

However, that copyright wouldn't protect the process or any other dissimilar design, and if you're planning on making many designs, it's not very practical to copyright each one. You should consult an IP lawyer about this art, since a copyright probably isn't the best way of going about this.

If you want to protect the process of making jewelry using metal clay, then that might be eligible for patent protection. Perhaps more appropriately, if you want to protect your brand as the source of all your jewelry products, then you want a trademark for the jewelry class of products.

Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.

Query: If I publish and start selling the pieces, does the fact that I already have filed for the copyright on it but not received the certificate bar anyone from stealing my idea?

Answer: The effective date of a copyright registration “is the day on which an application, deposit, and fee … are received in the Copyright Office.” 17 U.S.C. §410(d) (emphasis added). According to the statute, all rights arising from your copyright registrations relate back to the date each application was received.

If you want to protect your jewelry design you need to seek a Design Patent, as copyright does not protect your idea just its expression.
Contact a patent attorney, such as myself, to register your design.