Diamonds. There's only two reasons for a guy to buy 'em (and actually, the two reasons turn out to be the same reason -- oops, spoiler). The first is to show 'em off, to attract the ladies. The second is because a lady will ask you to, and if you don't, she'll probably stop having sex with you. So the only reason guys buys diamonds is 'cause girls want diamonds. And the only reason girls want diamonds is 'cause there's a conspiracy that brainwashed them into wanting it. Wikipedia says it, so it must be true.

Today, diamonds are used to symbolize eternity and love, being often seen adorning engagement rings and sometimes wedding rings as well. The popularity of this modern tradition can be traced directly to the marketing campaigns of De Beers, starting in 1938. Prior to the De Beers marketing campaign, engagement rings had no one particular stone associated with them.

Seriously though, there are a couple other sources which also claim the existence of this conspiracy.

Eighty years ago, your great grandparents didn't do this when they got married. They gave each other big wooden boxes and simple things like promise rings and hope chests. The allure of diamonds is part of a huge, century-long conspiracy by the diamond industry, namely giant De Beers, which controls stockpiles and sets the price of stones, which aren't the rarest in nature, even though they're the most expensive.

http://www.blacktable.com/bruno031030.htm

Until the late nineteenth century, diamonds were found only in a few riverbeds in India and in the jungles of Brazil, and the entire world production of gem diamonds amounted to a few pounds a year. In 1870, however, huge diamond mines were discovered near the Orange River, in South Africa, where diamonds were soon being scooped out by the ton. Suddenly, the market was deluged with diamonds. The British financiers who had organized the South African mines quickly realized that their investment was endangered; diamonds had little intrinsic value-and their price depended almost entirely on their scarcity. The financiers feared that when new mines were developed in South Africa, diamonds would become at best only semiprecious gems.

...

The diamond invention is far more than a monopoly for fixing diamond prices; it is a mechanism for converting tiny crystals of carbon into universally recognized tokens of wealth, power, and romance. To achieve this goal, De Beers had to control demand as well as supply. Both women and men had to be made to perceive diamonds not as marketable precious stones but as an inseparable part of courtship and married life. To stabilize the market, De Beers had to endow these stones with a sentiment that would inhibit the public from ever reselling them. The illusion had to be created that diamonds were forever -- "forever" in the sense that they should never be resold.

http://www.theatlantic.com/doc/198202/diamond

It's an interesting story. I wish I knew someone old enough (they'd have to be at least 69 by now) to confirm whether or not it was "expected" to buy a diamond ring for marriages.