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The $10K question: How to buy an engagement ring?

$ave me: The engagement ring

Typical holiday budgets go right out the window for consumers planning a Christmas or New Year's marriage proposal.

The holidays are the most popular time of the year to pop the question, with 16 percent of couples getting engaged during December, according to The average engagement ring tab last year was $5,431—although a 2012 survey of luxury jewelers by Centurion found that nearly 30 percent said $10,000-plus was their best-selling price category. (Almost 80 percent said their best-selling rings cost more than $5,000.)

Whatever you can afford, tweaking the four C's—that's cut, color, clarity and carat weight—can help you get the best stone for your budget. Jewelry experts recommend prioritizing cut, which is what makes the diamond sparkle (or not), and compromising on color and clarity if need be.

Doing so could bring a bigger, better stone into budget. On, for example, a 1-carat round stone with an excellent cut, VVSI clarity rating (one off from flawless) and D color (the highest) sells for as much as $15,698. Dropping down to an H color could save you $6,995 on a stone of the same size, cut and clarity, or let you trade up to a 1.46-carat stone of the same cut and clarity (that's still $66 less expensive).

It can also help to pay cash, and not just from the perspective of avoiding debt. Many jewelers offer a cash discount of roughly 10 percent.

By CNBC's Kelli B. Grant. Follow her on Twitter @kelligrant.