Diamond

• 1. Introduction

  ›

  Diamond is a mineral form of carbon whose atoms lock together in a structure so rigid that it outranks every other natural material in hardness and thermal conductivity. A billion years or more in the making, carried to the surface by rare volcanic eruptions, and named from the ancient Greek adámas, meaning unbreakable and untamed, it is both the world's most coveted gemstone and one of industry's most essential tools. How did carbon, the same element in a pencil's graphite core, become the hardest substance on Earth? Why do some diamonds glow blue or flush pink while others burn with a pale blue flame? And what does a mineral born deep in the mantle have to do with ocean currents on Neptune? Those are the questions this documentary will answer.

• 2. Ancient Beginnings

  ›

  Diamonds have been known in India for at least 3,000 years, and possibly as many as 6,000. Significant alluvial deposits lay along the rivers Penner, Krishna, and Godavari, where the stones could be gathered from sediments without any underground mining. From those riverbanks they moved into temples as religious icons, and into craftsmen's hands as engraving tools.

  India led global diamond production from around the 9th century BC all the way to the mid-18th century AD, a run of dominance that lasted roughly two and a half millennia. By the late 18th century, however, the accessible deposits were exhausted. The next chapter opened in 1725 when the first non-Indian diamonds were found in Brazil, shifting the center of the trade to a new continent for the first time in recorded history.

  In 1772, the French scientist Antoine Lavoisier focused sunlight through a lens onto a diamond inside an atmosphere of pure oxygen and watched it burn away entirely into carbon dioxide. That single experiment proved that diamond is made of carbon. Twenty-five years later, the English chemist Smithson Tennant closed the argument by showing that burning both diamond and graphite releases identical amounts of gas, establishing that the two substances are chemically the same material arranged in different ways. The word adámas, chosen thousands of years before any chemist weighed a combustion product, had nonetheless captured something true: diamond resists almost everything that would alter or destroy it.

• 3. Born in the Mantle

  ›

  Most gem-quality diamonds form at depths of 150-250 kilometers inside the Earth's lithosphere, beneath the stable ancient cores of continents called cratons. At those depths, pressures reach 4.5 gigapascals and temperatures around 950 degrees Celsius, conditions that make diamond the thermodynamically favored form of carbon rather than graphite. A fluid rich in carbon, oxygen, hydrogen, nitrogen, and sulfur moves through rock and, in a process called metasomatic replacement, dissolves existing minerals and deposits diamond in their place.

  The kimberlite eruptions that carry diamonds upward travel at 4-20 meters per second through hydraulic fractures, eventually exploding at the surface at speeds exceeding 200 meters per second. The kimberlites themselves are far younger than the diamonds they transport; most are between tens of millions and 300 million years old, while the diamonds they carry range from 1 to 3.5 billion years in age. No kimberlite has erupted in recorded human history.

  A smaller fraction of diamonds, roughly 150 studied examples, come from depths of 330-660 km, a region that includes the mantle transition zone. These stones carry inclusions of majorite, a form of garnet with excess silicon, which betrays their unusual depth of origin. In 2018, the first known natural samples of Ice VII, a high-pressure phase of water ice, were found trapped inside diamond inclusions, providing direct evidence of water-rich fluid deep in the mantle between 400 and 800 kilometers down.

• 4. Hardness and Its Limits

  ›

  Diamond sits at the top of the Mohs hardness scale, and the Vickers hardness test, which strikes a material with a standardized diamond pyramid, relies on this supremacy to measure everything else. The source of that hardness is structural: every carbon atom in diamond bonds to four neighbors in a tetrahedral arrangement, and diamond holds the greatest number of atoms per unit volume of any known substance. Tetrahedra are rigid, the bonds are strong, and nothing in nature compresses more reluctantly.

  Hardness is not indestructibility. Diamond can be split by a well-angled blow, and its toughness, measured at 50-65 MPa, is good compared to ceramics but poor next to most engineering alloys. Diamond cutters exploit the cleavage plane deliberately, sometimes splitting a rough stone before faceting it. The hardest natural diamonds come from the Copeton and Bingara fields in New South Wales, Australia, where single-stage crystal growth produces small, nearly perfect octahedra used specifically to polish other diamonds.

  At the nanoscale, diamond surprises again. Single-crystal diamond wires and needles in the range of 100-300 nanometers in diameter can be elastically stretched by as much as 9-10 percent without breaking, approaching the theoretical tensile limit for the material. Diamond anvil cells, exploiting the compressive yield strength of 130-140 GPa, have reached experimental pressures of 600 GPa; researchers believe nanocrystalline diamonds could push that ceiling considerably higher.

• 5. Color, Clarity, and Rarity

  ›

  Pure diamond transmits visible light and appears colorless because its band gap of 5.5 electron volts corresponds to deep ultraviolet light at 225 nanometers, well outside the visible range. Every departure from that colorless ideal traces to an impurity or a structural flaw. Nitrogen, the most common contaminant in gem diamonds, produces yellow and brown hues depending on its concentration. Boron produces blue, and also turns diamond into a semiconductor rather than an insulator. Radiation exposure from alpha particles causes green, while plastic deformation of the crystal lattice is responsible for some pink and red stones.

  In order of increasing rarity, the color sequence runs: yellow, brown, colorless, blue, green, black, pink, orange, and red. The Gemological Institute of America grades ordinary diamonds on a scale from D, meaning colorless, down to Z, meaning light yellow. Stones of unusually high color saturation or unconventional hue fall into a separate category called fancy colored diamonds. In 2008, the 35.56 carat Wittelsbach Diamond, a blue stone once owned by the King of Spain, sold at Christie's for over 24 million US dollars. The following year, a 5 carat vivid pink diamond sold for 10.8 million US dollars in Hong Kong on the 1st of December 2009, breaking the per-carat record set only months earlier by a 7.03 carat blue diamond that fetched 10.5 million Swiss francs.

  Between 25% and 35% of natural diamonds fluoresce under long-wave ultraviolet light. Blue is the most common fluorescence color, though orange, yellow, green, white, and very rarely red and purple also occur. The causes are not fully understood, but variations in the number of nitrogen atoms in the crystal structure are thought to play a role.

• 6. Industry and the Diamond Trade

  ›

  Eighty percent of all diamonds mined are unsuitable for gemstones and go directly into industry, where hardness and thermal conductivity matter far more than sparkle. Approximately 130 million carats are mined annually at a total value of nearly 9 billion US dollars. In 2014, a further 4.5 billion carats of synthetic diamonds were produced in a single year, with 90% of that output coming from China; synthetic material now accounts for roughly 90% of all diamond grinding grit used worldwide.

  The gem side of the trade is concentrated in a small number of cities. In 2003-92% of the world's diamonds were cut and polished in Surat, India. Antwerp handles 80% of all rough diamonds globally and more than 50% of all cut diamonds, earning it the informal title of world diamond capital. The Antwerpsche Diamantkring, created in 1929, was the first bourse dedicated exclusively to rough diamonds.

  De Beers, founded in 1888 by Cecil Rhodes, held a dominant grip on the industry for most of the 20th century, at one point handling more than 80% of the world's rough diamond supply. By 2013, its market share had declined to around 38% by value. The advertising firm N. W. Ayer & Son, retained by De Beers in the mid-20th century, revived the American market and created new markets in countries with no prior diamond tradition; the campaign's lasting artifact is the slogan "a diamond is forever." De Beers effectively discontinued the generic diamond advertising campaign by early 2011, shifting instead to promoting its own brands.

  Conflict diamonds, mined by armed groups in politically unstable parts of Africa to fund military operations, prompted the United Nations and the diamond industry to introduce the Kimberley Process in 2002. According to the International Diamond Manufacturers Association, conflict diamonds still constitute 2-3% of all diamonds traded, held in check but not eliminated by the certification scheme.

• 7. Diamond Beyond Earth

  ›

  About three percent of the carbon in meteorites exists as nanodiamonds just a few nanometers across. The isotopic signatures of some of those nanodiamonds show they formed outside the Solar System entirely, inside stars that preceded the Sun.

  High-pressure experiments predict that methane in the atmospheres of the ice giants Uranus and Neptune condenses into a diamond rain, producing large quantities of diamond that settle toward the planets' cores. Both Uranus and Neptune are composed of approximately 10% carbon. Results published in Nature Physics in 2010 suggest that at pressures around one trillion pascals and temperatures near 50,000 degrees Celsius, diamond melts into a metallic fluid. Because large quantities of metallic fluid can influence a planet's magnetic field, this process may explain why the geographic and magnetic poles of both planets are misaligned.

  One theory for the origin of carbonado, the toughest and blackest form of diamond, places its formation inside a white dwarf or a supernova. If that theory is correct, diamonds formed in stars may have been among the first minerals ever to exist. Back on Earth, the first double diamond, a stone containing a second complete diamond inside a cavity, was found by the company Alrosa in Yakutia, Russia, in 2019; it was named the Matryoshka.