Financing jewelry is almost always a bad idea. So much so that I hate to be writing a tutorial on the subject. Most people who are considering buying jewelry on time are young and inexperienced with debt and the affect that it has on their lives. The traps are huge and my first and most unambiguous piece of advise is:
Don't Do This!
If you insist on ignoring the above advise, There are a few ways to reduce the pain and to avoid the sharks. Here are a few pointers for the benefit of the public.
Many jewelry stores are what the industry calls Credit Jewelers. Malls in particular use this model. The idea is simple, you can buy a ring worth $xxx but you don't really need to come up with the money right now. You can make manageable monthly payments until it's paid off. If you've got a new job but haven't had a chance to save up much money yet this can seem like a pretty attractive deal. Get a nice ring, get married, advance up the career track, live happily ever after. Besides, you don't want her (or her friends and relatives) to think you're cheap.
For starters, understand that you are making two different transactions. You are buying a piece of jewelry and you are borrowing some money. They should be evaluated separately, even if they are happening at the same time and with the same company. Compare the jewelry with similar things offered at other dealers. In addition to the obvious gemological issues, many people put a high value on warranties and guarantees, return and upgrade privileges, status associated with the jeweler and many other things. This is discussed at length in the tutorial on buying jewelry. The subject here is financing so lets assume that you've narrowed down your shopping to exactly what you want. Now you're trying to decide how to pay for it.
It's pretty common for the offered financial arrangements to be pretty attractive because they have inflated the price of the jewelry in the first place. There's nothing particularly wrong with this as long as you understand what you're buying and what you're getting. If a comparable ring elsewhere (or even at that same store) is $1000 less without the financing, then you are paying a $1000 initiation fee for the financing in addition to whatever interest you will pay over the life of the loan. This is the reason for thinking about the two purchases separately. If you can get what you want for a piece of jewelry while paying less for the financing, obviously this is a good plan.
The popularity of online jewelers has added a new wrinkle to the decision. They usually will accept credit cards but they charge a slightly higher price if you choose to use one. Usually this premium is about 2% and it covers their fees to the card processing company. Most customers view this as a fee for the insurance more than a fee for the financing but you may wish to make it part of your calculations.