The power behind magical jewelry springs from three primary sources: the spell (infused into the piece by its creator), the symbolism (for example, the choice of colors or the use of sentimental items), and the innate properties of the materials used.
But the most potent power of magical jewelry lies in its ability to act as a tangible symbol, a magical sigil reminding the wearer of its magical goal. As the intent (both of the maker and the wearer) counts most, magical jewelry can be made out of most anything as long as the intent is there. Still, it is fun to draw inspiration from age-old traditions when creating your own personal spell. For that reason, start by doing a bit of research: grab your favorite magical encyclopedia and read through the tables of correspondence; check out all the elements of your potential creation, from the type of jewelry itself to the materials out of which you will construct it. What you are looking for isn’t just the one-and-only abracadabra material, but for what resonates with you (or, if you are creating a gift, with the intended recipient).