The universe is full of bright and shiny objects, including the occasional glittering space gem. Closer to home, luminous moons with ancient ice in their shadowed craters bounce light back at passing satellites whose solar panels seem to flash and flare when tilted at certain angles. Buried among these are a class of rarer objects that sparkle, but they aren’t stars, planets, or even spacecraft. Gemstones in space might not glitter when their ancient crystals of carbon, sand and metal reflect the light from nearby suns, but rare space gems found inside meteorites from India, Russia and Sudan provide us with an unparalleled view of processes at work inside distant worlds.
All That Glitters Is Not Gold
Humankind has long been aware that precious materials rain down from the heavens. The metal dagger buried with King Tut, for example, was made of “iron from heaven” — a blend of iron, nickel and cobalt found only in meteorites. It was prized by the ancient Egyptians as durable material for weapons and tools. To prove that the “iron-from-heaven” dagger really did fall from the stars, scientists at NASA’s Jet Propulsion Laboratory used X-ray fluorescence spectrometry. Fluorescence is a type of atomic fingerprint. Because every element has a slightly different configuration of electrons, every element absorbs and then releases energy a bit differently. To test what the dagger was made of without even touching it, scientists bombarded the dagger with X-rays then watched, using a detector, to see how the metal in the dagger would absorb then re-emit the energy. The dagger contained iron, 11 nickel and cobalt. What’s most important about this is that this particular combination does not occur on Earth.
Researchers can look for space gems in a similar way. Using X-Ray diffraction (which watches how a crystal scatters light instead of how a metal absorbs it), researchers discovered nanodiamonds in pieces of a meteorite that had exploded over Sudan in 2008. Nanodiamonds are crystals made of carbon. As is the case with Earth diamonds, space gem diamonds form in places without oxygen. The reason is simple: if you let the forming carbon crystals touch oxygen, they turn into graphite — the mineral used in pencils.
Natural Earth diamonds are formed 150 to 700 km down from the surface. Here, the carbon crystals endure immense pressure, and in the absence of oxygen, they form into their unique double-pyramid shape. These non-metallic carbon crystals are then carried to the Earth’s surface by volcanic eruptions or shifts in tectonic plates. Occasionally, erosion of land by rain, wind and rivers may reveal where diamonds were formed millions or even hundreds of millions of years ago.
Space gems — diamonds, sapphires, rubies and opals — may form the same way, deep inside faraway planets. If there’s no water or oxygen around, and the planet is massive enough to create crushing pressure, it is possible for a space gem to form in the atmosphere of a gas giant. This possibility has led to discussions about whether the weather on Saturn includes diamond rain. Carbon formed in the presence of oxygen in one part of Saturn’s atmosphere is crushed first into graphite and then into diamond as it falls toward the center of the planet. Diamonds formed in this way are hypothesized to be able to reach about a centimeter in size, uncut.
Because they form in gas and not rock, space gems born in the atmospheres of stars are typically tiny — so tiny that rather than falling inward they would be blown outward on solar winds. Such nanodiamonds have been found via microscopy in stony meteorites. Their double-pyramid crystal patterns prove that wherever they came from in the galaxy, there was a lot of pressure and little to no oxygen.
The space dust that streams down on us all day, every day, includes bits and pieces of all kinds of precious, semi-precious, and common materials made in the outer layers of our sun and billions of other stars. Larger diamonds also rain down on Earth from time to time. In 2021, NASA geologists found two 10-micron diamonds in a meteorite that fell in Sutter’s Mill, California. While these two were much larger than the usual nanodiamonds, 10 microns is still very small: about 1/10th the diameter of a human hair.
Diamonds are not the only gemstones in space. Opals — specifically fire opals — have been found on Mars. In stark contrast to how diamonds can be used as evidence of the absence of water and oxygen, opal crystals form in the presence of liquid. The discovery of fire opal crystals near a deposit of another substance, ferrihydrite, has been used as evidence that water and rock interacted for extended periods of time on the surface of Mars.
It would be quite something if the 50 tons of space debris falling on us every day carried with it diamonds big enough to see with the naked eye. Sadly, large space gems are much rarer than that, at least around here. However, nanodiamonds might well be a feature of every star with carbon in its atmosphere, giving the entire visible universe yet another reason to shine on.
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