Many witches and other magic-users wear jewelry for various purposes besides simply adornment. The pentacle worn by many witches symbolizes their beliefs and identifies them to others of similar belief systems. It's also a protective amulet. Other symbols such as Thor's Hammer and the triquetra serve similar purposes, but these aren't the only types of jewelry we witches wear.
Starting with the most basic, the wedding ring is a plain band. An unadorned circle with no beginning or end. There is also the promise ring, often with a Celtic knot design to symbolize two wondering paths that always converge, over and over through eternity, and, of course, the engagement ring with its diamond - the hardest mineral known to man. It can cut through anything, amplifies energy and is virtually indestructible - and don't we all hope our love will be the same.
Many of us also wear a quartz crystal wand on a chain. Quartz crystal wands are very useful in magic as they can be used to cleanse and charge other crystals as well as direct personal energy, so wearing one around your neck, you always have one handy if you need it. However, quartz crystals are also powerful amulets just hanging there minding their own business. They help keep the aura clear and strong, thus protecting the wearer from psychic attack and the affects of others' negativity.
Other crystals are often worn around the neck or wrist in the form of strings of beads. Depending on the size or shape of these beads and how they are strung, this can be a classic, even conservative look that also incorporates the magical energy contained within the crystals. Bloodstone makes a lovely bracelet and can be worn by women to ensure a healthy pregnancy. Beads of jet look quite elegant worn around the neck and protect against attacks of many sorts.
Some stones lend themselves well to being carved into shapes, such as rings and bangle bracelets. Agate can be worn this way to protect the wearer from contagion and poison - including the sort that comes from gossip and the negativity of others. Jade is another that can be carved into a beautiful piece of jewelry all its own for luck and love and it is said that wearing jade while gardening will increase the fertility of your garden plot.
Many stones are cut and faceted and set into metal rubies, amethysts and of course diamonds are some examples of these. Metals carry their own magical energies as well so consciously combining stone with metal can make for a powerful amulet.
You can find alot more information about metals, crystals and other minerals in the Witchipedia's crystal section. http://www.witchipedia.com/stones-minerals-and-crystals
Remember when you are wearing your magical jewelry that it doesn't work in a vacuum. You must help it along by cleansing it and charging it when its new and wearing it with intention. You must also behave appropriately. Do not expect your obsidian bracelet to protect you in your travels if you habitually drive in a reckless manner!