" " Some people might find it strange to spend a small fortune on an engagement ring. ©Brand X Pictures/Thinkstock

Today it takes the average would-be groom three months to find the perfect diamond engagement ring -- and it'll probably take him significantly longer to pay it off. The average cost of a diamond engagement ring is $5,200 for American brides (and as many as 12 percent of engaged couples admit they spent more than $8,000 just on the ring). And the most popular type? One with at least 1 carat for the center stone, in round or princess shape, set in a white gold band [source: XO group].

But before the 1900s, a diamond engagement ring was a luxury item; in fact, engagement rings even without a diamond setting were considered extravagant. The Archduke Maximilian of Austria gave one to Mary of Burgundy in 1477, but it was hardly commonplace. It was a diamond company that would change that. In the late 1800s, the diamond market was flooded when big diamond mines were found in South Africa, and that new influx of gems drove the prices down. In 1939, De Beers hired N.W. Ayer & Son as their ad agency to help them rebound from the slump in sales they'd had as a result of the South African mines. And a new wedding tradition was born: Diamond sales rose 55 percent between 1938 and 1941, and by 1948 America was introduced to the "A Diamond Is Forever" ad campaign -- and a new engagement tradition swept the U.S.

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