Under Section 102 of the Copyright Act, original jewelry designs are considered "pictorial, graphic, and sculptural works" and are protected by copyright law.
Whether it is worth it to register the copyright you have upon creation of the work is a different question. It's fairly inexpensive to do, and gives you certain rights which you would not otherwise have (such as the right to sue in federal court), but copy-catting is endemic in the world of jewelry design and policing violations is difficult.
Sorry, I missed the last part of your question. While jewelry with the symbol embedded in it would be subject to copyright protection, the symbol all by itself is something that would potentially be a trademark -- a sign that represents your company, perhaps -- or that identifies you, in the way that the symbol used by the-artist-formerly-known-as-Prince used to represent him. Realistically, the symbol would have to be unique or you would have to be so famous that everyone knew you by it in order for it to be protectable.
Jewelry is a product that can encompass multiple types of intellectual property rights, but these rights may vary depending on the particular countries in which protection is sought. In the U.S., jewelry or portions thereof may be entitled to copyright, trademark, trade dress, and design patent protection.
Copyright may protect the expressive/artistic component of the jewelry. But in the U.S., one does not need to register a copyright to be entitled to copyright protection. Registration entitles the owner to certain added benefits like possibly avoiding the need to show that the owner suffered financial losses as a result of another's unauthorized copying and being able to require the infringer to pay the owner's attorney's fees. It is relatively simple and inexpensive to obtain a copyright registration. See www.copyright.gov. The symbol, apart from its use on jewelry, may also be protectable under copyright as well.
As my colleague advised, it may be possible to obtain a trademark registration for the two or three dimensional symbol used on the jewelry because the symbol may be a way for consumers to identify the designer/owner as the source of the product. If one intends to use the symbol on other types of products, such as clothing, it may serve as a "source identifier" for those goods as well. In the U.S., one can acquire trademark rights through a federal registration, a state registration, and/or just using the symbol as a trademark for goods/services. A single federal trademark application can cover multiple types/classes of goods.
Also, the entire piece of jewelry or portions thereof may be protectable as trade dress (as a three-dimensional trademark) and/or via a design patent. In the same way that a brand name or logo may help consumers to distinguish the products from those of other manufacturers, the product or a portion thereof may itself be distinctive and protectable as a three dimensional mark. A design patent protects a novel, non-obvious ornamental design, and once obtained may also support registration as a three-dimensional trademark.
The above is intended purely as legal information and not as legal advice, and does not necessarily represent the views of the firm at which I work.