Why are diamonds so expensive if they aren't rare?

Diamonds are expensive because their scarcity was engineered, not discovered. Gem-quality diamonds are actually plentiful in the ground — the problem, from a seller's point of view, is that they're too plentiful. For most of the twentieth century a single company, De Beers, solved that problem by buying up the world's rough diamonds and releasing them in a controlled trickle, while its advertising made a diamond ring feel mandatory. High price is what happens when one hand controls the supply valve and the other writes the demand spec.

The supply valve

After huge diamond deposits were found in South Africa in the late 1800s, prices should have collapsed — suddenly there were far more stones than buyers. Instead, the mine owners merged into De Beers and routed nearly all rough diamonds through a single selling channel. At its peak in the 1980s the company controlled roughly 85% of the world's rough diamond supply. When demand dipped, it stockpiled stones rather than cut prices. Buyers only ever saw a trickle, so the trickle looked rare. The cartel's grip has loosened since the 1990s — its share fell below half by the mid-2000s — but the pricing habits it drafted are still the industry's blueprint.

The demand spec

Controlling supply only works if people keep wanting the product. In 1947 a copywriter named Frances Gerety, working for De Beers' ad agency N.W. Ayer, wrote four words: A Diamond Is Forever. The campaign, launched the following year, did two jobs at once. It installed the diamond engagement ring as the default symbol of commitment — before the campaign, it wasn't the standard. And "forever" quietly told owners never to resell, which kept second-hand stones from flooding back into the market and undercutting new ones. Advertising Age later judged it the best slogan of the twentieth century.

The stress test

Lab-grown diamonds are the live experiment that exposes the design. They are chemically identical carbon — same crystal, same sparkle — but nobody controls their supply, so competition does what competition does: lab-grown prices have collapsed and now run about 86% below natural stones, with a one-carat lab diamond selling for around $500. Same material, no cartel, no century of ads — and the price is a rounding error. That gap between a mined diamond and its identical twin is a fairly precise measurement of what you're really paying for: the story.