WORLD

A pink diamond just became the most expensive gemstone ever sold at an auction, fetching a record sale price of $71.2 million in Hong Kong. (Washington Post)

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.

Discussion Ideas

• The “Pink Star” diamond is the most valuable gemstone ever sold at 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.

• Most diamonds are clear or colorless. Why are diamonds such as the Pink Star pink?
  • Even geologists aren’t quite sure! Stay tuned.
    • Scientists think that colorless diamonds, in the extreme pressure of Earth’s mantle, may have undergone some kind of seismic shock that created or altered “twin planes” in the stones’ molecular structure.
      • “Twin planes” describe the alternating pink and clear zones on a pink diamond visible to scientists on a molecular level. “The twin plane itself should not give rise to color. But we think when those twin planes form, and slide back and forth, one against the other like a fault plane, that certain kinds of defects formed. The defects give us the pink color. But what we’ve not been able to do yet is find the specific kind of defect.”

• 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:
    • Blue diamonds contain trace amounts of the element boron in their crystal matrix.
    • Black-colored diamonds contain graphite in their diamond lattices.
    • Yellow-, orange– and brown-colored diamonds contain nitrogen in their diamond lattices.
    • Purple diamonds have a high hydrogen content and their crystal lattices experienced deformation.
    • Green diamonds were exposed to radiation, which impacted their crystal lattices.

• Are pink diamonds rare?
  • Yes. According to the largest producer of pink diamonds in the world, pink diamonds account for less than 0.01% of global diamond production.
  • The most common diamonds are cloudy-colorless. The rarest diamonds are dark pink or red.

• Where are pink diamonds extracted?
  • More than 80% of pink diamonds are extracted from the Argyle mine in Kimberley, Australia. (The Kimberley is full of interesting old rocks, it seems.) The Argyle, an open pit mine owned by Rio Tinto, is rumored to close within five years. Pink diamonds are also mined in South Africa, Tanzania, Brazil, Russia, and Canada.
  • The Pink Star was extracted “somewhere in Africa” by the De Beers Group of Companies, one of the world’s largest diamond businesses.

• 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 lamproites. 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.”

TEACHERS TOOLKIT

Washington Post: Rare ‘Pink Star’ diamond is now the most expensive gemstone in the world: $71.2 million

Nat Geo: What is the mantle?

Nat Geo: Why so blue, diamond? study guide

BBC: What makes pink diamonds pink?