Everything you've been told about the "engagement ring rule" is a lie.

I’m sure you’ve heard of a few great diamond myths over time.

Diamonds most definitely are a girl’s best friend, you absolutely should be spending up to three months of your salary on an engagement ring and diamonds? They last forever.

The message that isn’t communicated as frequently as those cliches is this one: the diamond movement was born out of one of the most savvy PR campaigns in history.

De Beers, the company that had the monopoly on the diamond market for decades, in 1938 decided it was time to get Americans invested in diamonds again. It was around the time of the Great Depression, families were struggling to put food in their mouths and diamonds, unsurprisingly, weren’t the priority.

In fact, The Herald Sun reported late last year than the 1930s, less than 10 per cent of couples proposed with a diamond. Today, in the US, that number is closer to 80 per cent.

Closer to home, on our own soil, Choice found that couples in Australia spend an average of $5000 on engagement rings. So how did De Beers manage to turn around their branding from one saturated in the idea of luxury to one that was focused on the everyday couple?

In 1938, De Beers turned to an advertising agency called N. W. Ayer to help market their product as not just an item, but as an idea, a concept, and a movement.

A widely cited piece on diamonds by Edward Epstein in the Atlantic in 1982 reported that N.W. Ayer noted in its campaign that “there was no direct sale to be made. There was no brand name to be impressed on the public mind. There was simply an idea — the eternal emotional value surrounding the diamond.” It’s a common advertising tactic used today, but at the time, it was revolutionary.

Essentially, De Beers built on this to infiltrate the mindset of young couples. With regards to engagements, diamonds were to be the status quo. Men were expected to buy them and women expected to wear them.

And, as you can glean from every engagement announcement flooding your news feed today, it worked. Diamonds are still the marker almost every engagement.

According to Price Economics, within three years, despite the Great Depression, diamond sales in the U.S. increased by 55 percent because of De Beers campaign. At the time, they settled on the slogan, "A Diamond Is Forever" to accompany their ads.  It has appeared in every De Beers engagement advertisement since 1948 and was voted ‘slogan of the century’ by Advertising Age.

Not only did De Beers give many-a-Hollywood celebrity diamonds to wear in public, but they also created that "three-month-salary-rule" I'm sure is familiar; the idea that a man should spend three months of his salary on an engagement ring.

Today, we assume that idea is one deep in the throes of tradition - one born from years of engagements and years of marriages. Instead, that too was too was the brain child of De Beers. How can they get people to spend more? First, they started with one month, then argued it should be two months. It did not take long for the company to leverage off their platform, and change their sell to three months.

It's an archaic idea, but one that's occasionally thrown around as a very basic benchmark. And men may not follow it to the tee, but there's something to be said about having a benchmark for spending so high.

And just to add salt into the wound? Not only is the demand for diamonds a marketing invention, but diamonds aren’t actually that rare. De Beers just had such a monopoly on the market, they were able to restrict the supply, keeping the price of diamonds so high.

So when it comes to buying a ring? Buy within your budget - not according to some marketing ploy.