[SciShow intro plays]

Michael: Diamonds: they’re shiny, tough, and an icon of marriage proposals. On a chemical level, a diamond is just a bunch of carbon atoms bonded together in tetrahedrons. This crystal structure is why diamond is the hardest natural mineral, and beats most man-made ones too! So, we know that diamonds are special, but where do they come from in the first place?

Diamonds are formed in really extreme conditions, which is why you might find them at the sites of asteroid impacts, and even on meteorites in space. But we think most natural diamonds were made millions, if not billions of years ago, in the splooshy molten rock deep within the Earth.

Over 160 kilometers beneath the Earth’s surface, the intense heat and pressure forced carbon atoms into that tetrahedral structure to form diamonds. Then, deep volcanic eruptions blasted them up to the surface, at around 40 kilometers per hour. If the diamonds came up much slower than that, the gradually-decreasing pressure would let the carbon atoms shift back into other stable forms like graphite.

At the surface, everything cooled really quickly to make these igneous rock formations called kimberlites, with diamonds hiding inside. And, eventually, we humans came along to mine them. So we know that diamonds are made of squished and superheated carbon atoms, but we’re not exactly sure where that carbon came from.

Geologists have two main guesses: For one, there could be some kind of carbon-containing sediment deep within the Earth’s mantle, that never saw the light of day. Or maybe the carbon in diamonds came from organic stuff in subducting tectonic plates, like stuff on the ocean floors, but not coal. That whole “all diamonds are just coal under pressure” thing is mostly just... wrong.

See, coal is a form of organic carbon, but it was made mostly out of ancient plants, in layers of sedimentary rock closer to the Earth’s surface. So it formed way later and way higher up than we think most diamonds did. These days, scientists can also make synthetic diamonds in a lab with a process called chemical vapor deposition.

Basically, they start with a tiny seed of natural diamond to provide a “pattern.” Then, they use high pressure and temperature tools to turn carbon atoms, usually from a carbon-containing gas, into layers of diamond. These lab-made diamonds are generally flawless, unlike many natural ones.

Plus, they can be cheaper to buy, and have less of an environmental impact than mining natural diamonds. So whether a diamond came from deep within the Earth or a materials science lab, it’s still the same mineral, just with a different story.

Thanks for asking, and thanks especially to all of our patrons on Patreon who keep these answers coming. If you’d like to submit questions to be answered, or get some videos a few days early, go to Patreon.com/SciShow. And don’t forget to go to YouTube.com/SciShow and subscribe!