Why So Blue, Diamond?


One of the largest blue diamonds in the world is coming to auction, expected to fetch more than $30 million. (Forbes)

Diamonds are a geologist’s best friend! Use our resources to find out why.

Teachers, scroll down for a quick list of key resources in our Teachers’ Toolkit.

Behold the Millennium Jewel 4. This oval-cut, 10.10-carat blue beauty is flanked by two pear-shaped colorless diamonds and diamond band set in 18-karat white gold. Photograph by Sothebys
Behold the Millennium Jewel 4. This oval-cut, 10.10-carat blue beauty is flanked by two pear-shaped colorless diamonds and diamond band set in 18-karat white gold.
Photograph by Sothebys

Discussion Ideas

  • The “Millennium Jewel 4” is one of the most valuable diamonds ever to come to auction. What is a diamond?
    • Diamonds are carbon: Diamonds are allotropes of the element carbon. Allotropes are simply different forms of the same element; graphite and carbon nanotubes are other carbon allotropes. The carbon atoms in diamonds are arranged in a specific crystal structure called a diamond lattice.
    • Diamonds are rock-solid: Diamonds are the hardest natural substance in the world. Many diamonds can only be scratched by other diamonds—and they’re too tough to evaporate or melt!
    • Diamonds are a girl’s best friend: Diamonds are among the most valuable of all gemstones. According to About, nine out of every ten dollars spent on jewelry in the U.S. is spent on diamonds.
      • FYI: The world’s most expensive diamond was purchased for $48.4 million last year by Hong Kong real estate tycoon Joseph Lau. Lau bought the jewel for his 7-year-old daughter—for whom the stone (the “Blue Moon of Josephine”) was named.


  • Most diamonds are clear or colorless. Why are diamonds such as the Millennium Jewel 4 or the Hope Diamond blue?
    • Blue diamonds are, technically, defects. An imperfection in the mineral’s diamond lattice allows trace amounts of the element boron to enter the crystal matrix.


  • Do diamonds come in any other colors?
    • Yes. Different chemical impurities and defects in the diamond lattice change the visible absorption spectrum of the mineral. Some examples:
      • Black-colored diamonds contain graphite in their diamond lattices.
      • Yellow-, orange– and brown-colored diamonds contain nitrogen in their diamond lattices.
      • Red– or pink-colored diamonds contain no chemical impurities, but their diamond lattices deformed due to temperature and pressure.
      • Purple-colored diamonds have a high hydrogen content and their crystal lattices experienced deformation.
      • Green-colored diamonds were exposed to radiation, which impacted their crystal lattices.


  • Are blue diamonds rare?
    • Yes. According to Forbes, blue diamonds accounted for fewer than .1% of diamonds recovered at the mine where the Millennium Jewel 4 was extracted.
      • The most common diamonds are cloudy-colorless. The rarest diamonds are red.



  • Diamonds have been used in jewelry for millennia. What else are diamonds used for?
    • Diamonds’ hardness make them valuable industrial tools used for cutting, drilling, and grinding. (In fact, up to 80% of mined diamonds are used for industrial, not decorative, purposes.) Drilling equipment and scalpels may have diamond bits and blades. Industrial saws may use diamonds as cutting surfaces, while diamond powder is sometimes used as an abrasive or grinding agent.


  • Early on, we mentioned that diamonds are a geologist’s best friend. Why? Skip down to the section on “xenoliths” in this encyclopedic entry for some help.
    • According to our entry, “diamonds form under very unique conditions: in the upper mantle, at least 150 kilometers (93 miles) beneath the surface. (Above that depth and pressure, carbon crystallizes as graphite, not diamond.) Diamonds are brought to the surface in explosive volcanic eruptions, forming ‘diamond pipes’ of rocks called kimberlites and lamprolites. The diamonds themselves are of less interest to geologists than the xenoliths some contain. These intrusions are minerals from the mantle, trapped inside the rock-hard diamond. Diamond intrusions have allowed scientists to glimpse as far as 700 kilometers (435 miles) beneath Earth’s surface—the lower mantle. Xenolith studies have revealed that rocks in the deep mantle are most likely 3-billion-year old slabs of subducted seafloor. The diamond intrusions include water, ocean sediments, and even carbon.”



Forbes: 10-Carat De Beers Millennium Blue Diamond Could Fetch $35 Million

Sotheby’s: Magnificent Jewels and Jadeite—Superb and Rare Fancy Vivid Blue Diamond and Diamond Ring

Wikipedia Featured Article: What is a diamond?

Nat Geo: What is the mantle?

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