20 Diamond Facts Most People Have Never Heard

Here is a quick test of how well you know diamonds: they do not come from coal, most of them are older than the first plants that ever grew on land, and scientists have watched diamond "rain" form in a laboratory experiment designed to mimic the inside of Neptune. Diamonds sit at the crossroads of astronomy, deep-earth geology and human history, and the true stories are stranger than the marketing. Below are twenty diamond facts we find genuinely surprising — every one checked against reputable sources, and hedged honestly where the science is still settling.

Deep space

Fact 1: It may literally rain diamonds inside Neptune and Uranus. Thousands of kilometres down, pressure is thought to squeeze carbon out of hydrocarbon mixtures and crystallise it, so diamonds sink slowly towards the core like glittering hail. In 2017, researchers used a giant X-ray laser at the SLAC National Accelerator Laboratory in California to recreate those conditions and watched nanodiamonds form in real time — turning a long-standing hypothesis into an observed process.

Fact 2: There is a star with a crystallised carbon core. BPM 37093, a white dwarf about 50 light-years away in the constellation Centaurus, was nicknamed "Lucy" after the Beatles song when astronomers concluded in 2004 that much of its interior had crystallised. By one popular estimate, its core would weigh ten billion trillion trillion carats — though calling it "a diamond" is a poetic simplification of some very exotic physics.

Fact 3: Our own Sun is expected to follow. Billions of years from now, once the Sun has burned through its fuel, swollen, shed its outer layers and collapsed into a white dwarf, astronomers expect its carbon-rich remnant to crystallise the same way. The solar system may end with a diamond at its centre.

Deep earth

Fact 4: Diamonds do not come from coal. Coal forms from buried plant matter in the upper crust; diamonds crystallise from carbon far deeper in the mantle. More decisively, nearly every dated natural diamond is older than the first land plants — so the raw material for coal did not yet exist when most diamonds formed.

Fact 5: Most natural diamonds are between about one and 3.5 billion years old. The oldest dated examples, from mines in northern Canada, formed when Earth was barely a billion years old itself. The stone in a vintage ring may be a quarter of the age of the universe.

Fact 6: They reach the surface by volcanic express lift. Diamonds form roughly 150 to 200 kilometres down and ride up in rare, violent eruptions of a magma called kimberlite, which must travel fast enough that the diamonds do not revert to graphite on the way. No such eruption is known to have been witnessed in recorded human history.

Fact 7: A diamond is chemically the same element as a pencil lead. Both are essentially pure carbon; the only difference is how the atoms are bonded. Squeeze graphite's flat sheets into a rigid three-dimensional lattice and you get the hardest natural material known. You can read more about the underlying chemistry in our guide to how lab-grown diamonds are made.

Strange science

Fact 8: Only a diamond can scratch a diamond. It sits at 10 on the Mohs hardness scale, and nothing else in nature comes close — which is why diamond cutting and polishing is done with diamond-coated tools and diamond dust.

Fact 9: And yet diamonds can chip. Hardness (resistance to scratching) is not toughness (resistance to breaking). A diamond has cleavage planes — directions of relative weakness — and a sharp knock at the wrong angle can chip a girdle or split a stone. Historic cutters exploited exactly this to cleave large roughs. Any honest jeweller will tell you: diamonds are forever, but they are not indestructible.

Fact 10: Diamonds burn. Being carbon, a diamond ignites at roughly 900°C, and in a stream of pure oxygen it will burn away entirely, turning quietly into carbon dioxide. House fires occasionally damage diamonds this way — another reason insurers take jewellery seriously.

Fact 11: The Hope Diamond glows red in the dark. After exposure to ultraviolet light, the famous deep-blue stone phosphoresces a fiery orange-red for several minutes. Smithsonian researchers traced the effect to the interplay of boron and nitrogen in the crystal, and found the glow is so distinctive it can help fingerprint individual blue diamonds.

Fact 12: Red is the rarest diamond colour of all. Fewer than thirty true fancy red diamonds are thought to exist, and the largest one ever graded by the GIA — the Moussaieff Red — weighs just 5.11 carats, cut from a rough found by a Brazilian farmer in 1990.

Human history

Fact 13: The word "diamond" comes from the Greek adamas — roughly "untameable" or "unconquerable". The ancients believed the stone could not be broken and, by extension, could protect its wearer. The English word "adamant" shares the same root.

Fact 14: "Carat" comes from carob seeds. Ancient traders used the seeds of the carob tree — reasonably uniform at around 0.2 grams — as counterweights for weighing gems. That trader's habit survives in every certificate written today: one carat is still exactly 0.2 grams.

Fact 15: The largest gem-quality rough diamond ever found weighed 3,106 carats. The Cullinan, unearthed in South Africa in 1905, was presented to King Edward VII and cleaved into nine major stones; the two largest now sit in the British Crown Jewels. Its story sits alongside others in our tour of the most famous diamonds in history.

Fact 16: The Regent Diamond survived the 2025 Louvre heist untouched. When thieves used a furniture lift to raid the Apollo Gallery in October 2025 and escaped with eight pieces of the French crown jewels, they left behind the 140-carat Regent — a stone valued at a reported $60 million, more than most of the individual pieces they did take. Nobody knows quite why; some point, half-seriously, to its legendary curse. That is legend, not fact — but the diamond's escape is a matter of record.

Fact 17: Around 90% of the world's diamonds are cut and polished in one Indian city: Surat, in Gujarat. Whatever the origin of a stone — Botswana, Canada, Russia or a growing machine — the odds are its facets were placed by hands in Surat. It happens to be our home town, and where every Signora piece is made.

The modern chapter

Fact 18: The first verified lab-grown diamond was created in 1954. After years of failed attempts under GE's secretive "Project Superpressure", engineer H. Tracy Hall succeeded on 16 December 1954 — working alone, days before Christmas, on a 35-year-old press he had rebuilt himself — at enormous pressure and around 1,600°C.

Fact 19: In 2018, the US Federal Trade Commission removed the word "natural" from its definition of a diamond. A diamond, in the FTC's revised Jewelry Guides, is simply a mineral of essentially pure carbon crystallised in the isometric system — a definition a lab-grown stone meets in full, provided sellers clearly disclose its origin. The chemistry was never in dispute; the language finally caught up. We unpack the differences honestly in lab-grown vs natural diamonds.

Fact 20: Lab-grown centre stones now account for 61% of new engagement rings in the United States — roughly six in ten, according to The Knot's 2026 Real Weddings Study. What was a curiosity in 2015 became the mainstream choice within a decade, part of a longer story we trace in our history of diamond engagement rings.

Record holders at a glance

If there is a thread running through all twenty facts, it is that a diamond's real story needs no embellishment — the physics and the history are remarkable enough on their own. That is rather how we feel about our work at Signora. Every piece we make is crafted to order in Surat, the city behind fact seventeen, set with lab-grown diamonds that are IGI-certified with the report number laser-inscribed on the girdle, and delivered to the US, UK and Canada with duties prepaid in about two weeks. If the science here has made you curious, our engagement ring guide is a calm place to start.