Coins have been used as part of jewelry perhaps for as long as coins have existed.
Today, many fine examples of coin jewelry abound, using both ancient and modern coins in precious and base metals as part of the design for necklaces, earrings, rings, cufflinks and more.
The use of ancient coins in jewelry poses somewhat of a quandry to an ancient numismatist: While modern coins are extremely plentiful and minted by the millions, ancient coins are a scarcer resource and are obviously not being produced anymore.
Jewelry featuring, for example, American Gold Eagle coins, while it eliminates the numismatic value of a bullion coin, the final product exceeds that base value and does not really affect the cost of other American Eagle or other gold bullion coins, as they are so plentiful.
But, when a highly artistic ancient coin of exceptional quality is mounted in jewelry, the numismatic value of it is lost and it also affects the price of other similar ancient coins by making that type scarcer. Mounting such a piece may also cause it to lose relative value over time as high-grade unmounted coins tend to appreciate in value.
On the other hand, ancient coins that are plentiful and below the high-end collector grades are often made into jewlery and the product often exceeds the numismatic value of the coin by a considerable margin. A common, worn coin costing perhaps $30 at a coin show or from a dealer's bargain list may be mounted in a precious-metal jewelry item and sold for five or even ten times its numismatic value, as the resulting product turns an otherwise unexceptional coin into a feature of a beautiful new creation.
As a numismatist, I of course do not want to see the high grade, collectible coins mauled and formed into jewelry so that their intrinsic value is reduced. But, I have no problem with lower grade, more common coins being turned into artistic pieces for sale. Such jewelry offers buyers unique products and also makes ancient coins visible and interesting to the public. Then again, if someone wants to treat their property in a fashion differently than I would prefer, who am I to decide for them?
A good place to look for and at examples of coin jewelry is
Porat Jewlry which has a nice selection of coin-based jewelry as well as other products. I've ordered other jewelry (I haven't bought any coin jewelry myself yet) from Porat and was very pleased with the product and service. Their coins fit my definition of commoner types, so I have no quandry in directing you there to look and enjoy, and perhaps to buy and wear (note - Coin Jewelry is a very unique gift for Valentines Day that may get you in the good graces of your significant other).